I'm often asked why I play old RPGs instead of whatever the current fad is or whatever the latest edition of the game I'm playing is. Of course, most people asking are really asking why I have no interest in the latest version of D&D. In any case, my answer is really simple. I play games I own and enjoy.
I'm more likely to try a new game if it isn't trying to replace an old favorite, especially if it is doing so by trying to take an old favorite in directions I do not enjoy. D&D started going in directions I did not enjoy much with 2e. After a few years, I even stopped buying 2e stuff.
3e looked great at first, then I discovered how long combat was and how character build-oriented it was quickly becoming -- two things I loathe in RPGs. I dropped it quickly. I tried 4e but immediately discovered that it was even more into long detailed combats than 3.x was, that 4e rules actions did not map to reality very well -- and the damn build-orientation was still there, although toned down.
New ceased being better in the D&D world for me about 1992/1993 because the designers moved the newer versions game further and further away from the type of play I enjoyed into types of play I did not find fun at all. However, I have all this OD&D, AD&D1e, B/X, BECMI/RC D&D (and even some AD&D2e) stuff that still plays well and caters to the style I enjoy, so I play it. I really can't imagine why this surprises so many people. I don't waste a lot of time and money on all sorts of different D&D-like games in the hope that one of them will be better for me than what I already have, know well, and really enjoy.
Note that I'm like this with everything. I drive a 1998 minivan and 1982 car. They both work well and do their job, so why waste money on new ones just because they exist and many people think they are much better (for what they value as "better") than what I have? I guess I lack the "newer must be better" gene or something. I know friends in marketing and advertising have told me many times that they are glad most people aren't like me. :)
That said, I do pick up new games every once in a while. They are usually games for subjects I think I might enjoy but do not already have a "go to" game for -- or a system that appears to have a new and interesting (to me) take on a subject I really like. I buy based on what interests me, not on the need of some game company to put out a new version or whatever. Also, I seldom buy sight unseen any more. If I can't sit down and look through the game rules for an hour or so before purchasing it, chances are pretty good that I'll just decide to save my money rather than chance wasting it on yet another game that sounded good but turned out to be something I would probably never play more than once or twice.
However, it all boils down to I play what I like and I don't care if what I like isn't that popular, that new, or even that good in the eyes of some game design theorist.