It’s an interesting question. 

One related question that must be answered is why there seems
to be some perfect gateway games out there, perhaps hundreds or
even thousands of years old, which are played by quite a lot
of people yet aren’t gateway games---at least, not to the wider
world of boardgaming.  These include: chess, checkers, bridge
and other card games, even dexterity games like Crokinole.  In
much more recent categories, they can also include Magic the
Gathering and other CCGs (a small percentage seem to get involved
with games, but not so many) and others.  There are many gamers
who start with these and even continue playing them, but there
are many more people out there who don’t.

Perhaps the key to why the older games don’t spark interest like
the newer games is that they seem too routine.  Yet they are
indeed routine enough to be played by anybody; very mainstream,
and it’s not like you’d find a person that has never heard of
them, or even played them.  Some are too light to really express
the interesting possibilities of boardgames (which is probably
why good gateway games are rarely light a la 6 Nimmt/Category 5).
And some are so deep that it may escape the grasp of your average
Joe or Jill who doesn’t want to spend a life mastering a game
that they couldn’t be naturals at (or don’t believe they could be).

As to the newer games, they are different.  They aren’t mainstream,
yet they have an approachable quality.  They offer intriguing glances
at a world where rolling dice has a different purpose than simply
walking around the board.  Indeed, dice are familiar, and cards are
too---the element of chance is common among some of the more popular
games.  Probably they help “even up the odds” against even skilled
players---even if the skilled players would usually win.  For games
like chess you have no one to blame but yourself, and indeed, the
learning curve of strategy for these games is so daunting that without
a little luck it seems impossible.

People tend not to like spending time on the impossible.

People also like simple games, which is another quality shared with
popular games (except perhaps Monopoly and the intricacies of the
different variety of cards in CCGs).  Even gamers like simple games. 
It adds an extra layer of approachability---smooths out that learning
curve, even if it has the same basic difficulty to learn as chess or
bridge.

Gateway games also have a much shorter learning curve than some of
the popular heavies.  After a couple of games, you feel like you have
more of a grasp without having to resort to books and so on.  (Indeed,
one of Go’s tenants is that you should go out and lose your first 100
games as quickly as possible for the learning experience).  Yet there
is something to think about.  Even Carcassonne has something to think
about---yet not too much.

Summary: simple, short, and to the point in the dimensions of the
learning curve (rules and strategy).
That’s a good gateway game.

I agree with Rick and Ava.  A good gateway game is something that is not too daunting in its learning curve for a non-gamer to want to learn while being challenging enough for a gamer to want to teach. 

It’s also very important for the gamer-in-training to feel that he/she has a fighting chance, thus the luck element.

And to be honest, it’s more fun for me to introduce them to games where I don’t have a huge advantage due to my experience with a game.

As for why the CCG crowd rarely makes it into the boardgame crowd, I would suggest price.  Many CCG players won’t bat an eye at spending $5 to $10 a week slowly gathering cards, but would be quite hesitant to plunge into the unknown and plunk down $40 to $60 all at one time for a single board game.  (Hmm, perhaps that just shows a lack of forward planning that would just be exposed once they start playing boardgames!)

Yup, price might have something to do with CCGs, but I know many who don’t bat an eye at spending $140 dollars on two boxes of boosters.  CCGs aren’t cheap no matter how you cut it, and they know it more often than not.

I think maybe a contributing factor is that CCGs are all-consuming---because the metagame constantly changes, new expansions are always coming out, they eat your money, they eat your time (because you always need to spend time constructing decks, which is half the fun of a typical CCG).  And as any former CCG player can tell you, there’s just something about that ever-changing atmosphere that is entirely addictive.  The collection aspect probably has a lot to do with the attraction, too.

In fact, CCGs can narrow your thinking about games to the point that a game without a CCG metagame can seem boring because you no longer think that way (or haven’t for a while).  It wasn’t until I was off CCGs for a year or so that I got interested in boardgames.

People who fall out of the CCG loop seem to switch either to draft (which compresses the deck-building time and gives you some limits in which to express a bit of creativity---kind of bragging rights if you get through a tournament :)) or to pre-constructeds.  Unfortunately only a very few CCGs have good pre-constructeds (and I think MtG only started having good pre-constructeds when they had enough playtesters to test them out).

And that is why I tell people that real CCGs are way more consuming than things like Blue Moon can ever be.  In fact, I really like Blue Moon because (especially with the extra deck-building-focused expansions) it can capture almost all the deck sterotypes I ever found in CCGs.  Whereas in MtG, I would always have to change what a White Weenies or Big Green or Blue Control would be, and then with new cards there are new types to try out.  It is really hard to kick that addiction, and Blue Moon has helped out a lot for me in this area.

Still, I have something like 6 different CCGs still sitting in my house (including some older MtG rares) that I have never been able to get rid of.... even though I know I should....

There’s one drawback to Settlers and Carcassonne in particular: the rules are actually somewhat unintelligible for a first time gamer (any non euro gamer) to grasp by reading the enclosed ruleset. Days of Wonder does a much better job and their layout and design to make rules a pleasure to read in comparison.

I learned both Settlers and Carcassonne by reading the rules, having only been exposed to Elfenland in terms of Euros previously, and it was quite the challenge. I recall that it took my best friend and I quite a bit of time navigating between the two Settlers rulebooks to figure out what was supposed to happen.

On the flip side, teaching both games is a treat, especially Carcassonne (and even better if you do as I do, and omit the farmer rule for folks who seem a little gunshy of having to learn a new type of game). Ticket to Ride is also very easy to teach.

This does make me wonder how many potential euro gamers are “lost” because of less-than-optimal rules for gateway games.

Speaking of rules…

I am a former CCGer and I think the immense and ever changing rule set of that world prepared me very well for Euro-gaming.  In particular I was left well equipped to navigate even the most treacherous rule book.  After dealing with dozens of syntactical interpretations and subtle cause and effects in CCGs, rules in general become easier to digest.

This is obviously true of reading rules in general, and also playing games in general.  However, this does harken back to the above discussion of CCGers converting to boardgames. There is at least one wall already broken down for those that attempt the move.

Another big wall is one’s resignation that they are in fact a geek… of one variety or another ;)

Good point, Ted.
I received Settlers (German version) from my wife for Christmas some years back, but was too intimidated by what looked like--at first glance--rules that were too complicated.

It was at least half a year before friends of ours taught us their copy of the game, and I realized that it wasn’t at all difficult to learn.