To summarize some things that have been said already, and to add to them as well:
1E DnD had a certain quirky charm. It was written by a single author who had a very distinctive style. There were lots of interesting, if esoteric and idiosyncratic, elements to it (I still enjoy reading the descriptions of the "artifacts and relics" in the DMG). The spirit of Vance, Leiber and Howard came through somehow in those pages. There was an exciting "newness" to it: the first mention of those mysterious Drow in "Against the Giants," etc. Also, it was pretty brazen for its time: I remember looking a little too often at certain pictures in the original Deities and Demigods

. It had Demons and Devils, baby! And Assasins. And REAL Illusionists! And the legendary "Master of Flowers."
2E DnD fixed up the rules SOMEWHAT. But it sacrificed a lot of the charm of 1E (and other features as well, like Monks, Assassins, etc).
It was a slightly "cleaned up" and -- unfortunately -- "sanitized" or "politically correct" version of 1E. (No demons or devils! What happened to Asmodeus, ruler of HELL?!

No more Erol Otus.)
I didn't really play DnD much during the 90s. 2E was one big reason.
3E DnD has a much more coherent and consistent set of rules than either 1E or 2E. It lacks the charm of 1E, IMO, and seems to promote a focus on "the rules" that is unfortunate. Its rule books also seem too slick for their own good. But it is hard to deny the superiority of the rules. They simply make more sense than 1E or 2E, and afford greater flexibility to players and DMs. (Now, if only combat didn't take so damn long...)
In short: 1E has a certain quirky charm to it (hence its fans), whereas 3E is both slick and coherent (hence its current popularity). 2E lacks either of these features. Consequently, 2E has few advocates. 1E is the original Atari; 3E is the X-box. 2E is Colecovision.