Fans of the Silver Age waited and wanted and hoped for the multiverse to come back, even though the whole point of the original Crisis was to get rid of it (and all that confusing history). Instead, all that DC managed to accomplish was making its single new continuity even MORE confusing, in part because no one seemed to be able to agree what the new story was going to be. (Part of the old multiverse's job was to allow for such variance between artistic vision.)
There were a number of attempts to bring it back, partly inspired by rosy nostalgia but also by the awareness that as a matter of practicality, you can't have "all stories are true" in a setting with so many creators and so much scope... but like the various incarnations of Supergirl, people kept tripping over each other trying to do it "right" according to their ideas. Thus you got stuff like Hypertime, 52, etc. By now it's gone through so many reinterpretations that it's hard to say just what the structure of the DCU cosmos is supposed to be. It's the same problem bumped up to the next level of meta - everyone wants to tell THE DEFINITIVE STORY of how all stories are possible.
Moral of the story? "Too many cooks spoil the broth" and/or "be careful what you wish for," I'd say.