When did this all start, and what is the appeal?
I have a long blog post about this here...
Uhluht'c Awakens: The Old-school "Revival": Origins, Current State, and Future; An open discussion
There are a number of factors. Others have mentioned the "genealogy", but I think there are other important issues. I'll just summarize a few I think are most important. Ironically, the push to associate 3.x with older styles of play served to sort of seed this idea that older play styles are worth preserving. "Third edition rules, first edition feel" pretty much summarizes how the idea of first edition as something positive was nurtured. I see this as one of the main factors for why retro gaming is starting to "leak" out to people who are too young to have been exposed to older games.
As a completely separate component, we have always had communities online that support older versions of D&D. I think that when people saw how the OGL was being used, to create entirely separate and new games, at some point people realized that the system reference documents contained all the IP necessary to emulate older rules, and without having to change a bunch of terms since they were available under license.
I think it's possible that the retro-clones might have languished as oddities were it not for the "first edition feel meme" and two very important events. The first event was our tragic loss of Gary Gygax. As everyone knows, Gary was very active on these boards and other boards. People, and I mean "modern gamers", on these boards knew of him; he was part of the online gaming community. When he died I think the sense of loss only naturally impelled people to revisit Gary's earlier work. There was a surge of old-school oriented blogs, and people also found out when they researched Gary's earlier work that there was already a bunch of retro-clone games out there to be discovered, currently supported, with growing fan bases.
The second event was the release of the 4th edition of Dungeons & Dragons. Who knows whether its release roughly coinciding with Gary's passing was a factor, but one thing that does seem to be clear is that a decent proportion of people are choosing not to switch to 4e. All of the various reasons for this I could speculate about, but the final result is that after all these years people are starting to realize that New Edition != More Evolved, or "better."
I'm a big fan of retro console gaming. I Love the NES, SNES, Sega Master System, Sega Genesis, etc. However, I cannot deny that later consoles did improve on graphics quality. None-the-less, I still prefer the older consoles because of the game style, and there is a charm to those old graphics!
I bring this up even though I risk confusing the point I'm working toward. The point is that p-n-p rpgs are not like console games. You can't measure "improvement" like you can when you go from 8 bit to 16 bit. Even that, which seems on the surface to be a quantitative improvement, does not necessarily equal a qualitative improvement. With p-n-p games, there is no hardware to improve so that the software can improve. Paper has pretty much been paper for a long time now, so p-n-p games change for cultural, political, personal, and other reasons. The business model that has prevailed for a long time is that the customers must believe a new edition is "needed" so that sales of core books can be revived and all the supplements resold as revisions for the new system, etc.
I'm not criticising that model, it's just business, but clearly that idea is starting to break down. One thing I think is important to say is that, as far as I know, none of us who have released retro-clone games wishes failure on the current edition of D&D. We just want the games we like to be supported; we want to share them, and we don't want them to be left to obscurity simply because at a point in one company's history they were deemed unprofitable. These works are all labors of love.
Anyway, I've rambled all of the place more than I meant to!