The OSR exists because of three things.
1) The older editions are found by many to be fun to play
2) The Internet makes it a lot easier to learn how other used older editions in their campaigns
3) The rise of digital technology like print on demand, desktop publishing has dropped cost of publishing professional quality material dramatically.
4) The core material has been recreated as open content.
5) Classic D&D was the world's most popular RPG by an order of magnitude while in active publication. This is where the "nostalgia" factor comes in as unlike Fate there a large potential fanbase who had enjoyable experiences with these editions.
And keep in mind that in terms of sales and raw number, I doubt the OSR AS A WHOLE is any larger than any RPG publisher that not in the top five in sales or audience.
What sets the OSR apart compared to other RPGs by other publishers is that it the IP is not controlled by a single publisher. Instead it is a kaleidoscope of one-man, small, and medium sized publisher putting out material on a regular basis. The only universal reason is an interest in a classic edition of D&D either 'as is' or in a modified form. Beyond that you can't make any predictions or observation because all about the specific circumstances of a individual publishers.
My circumstances are not the same at that of James Raggi nor Bill Webb, Zak S, Kevin Crawford, Tim Shorts, Jeff Rients, Erik Tenkar and dozens of other authors and contributors. What important to remember that ANYBODY can use the tools that are available to realize their vision of what one can do with classic D&D. That the only effective limitation is the time one has to devote out of their hobby.