Why everybody in Bleach can fly now?

[Necromas]

-Spoiler Removed-

Please make sure what you're recalling is from the anime and not the manga
- Assassin

Well it's basically the same thing, the reason it isn't done much before now is simply because it's a technique only experienced shinigami can do, and it's probably harder to do in soul society for some reason to cover the plot hole.

I believe Rukia DID float in the very first episode, but it's been ages since I've seen it so I could be wrong.