Is it really so baffling that the all-time simplest, easiest-to-learn, and easiest-to-DM edition of D&D that could still reasonably be called "complete" thanks to the level range it covers has remained the OSR's darling? After all, what are the alternatives? It won't be the white box, that's too rough around the edges. It won't be AD&D, that version's too complex. It won't be some mini hack like Knave or Maze Rats or Whitehack or the Black Hack or Into the Odd, because they're not D&D (by which I mean, they're not widely perceived as complete, D&D-equivalent systems in the same way that retro-clones are), and plenty of gamers will never even spare a glance in their direction for that reason alone.