The earthbones are literally Nirn; the Aedra are more like principles that govern Mundus as far as I understand (one of the loremasters will correct me if I don't understand, hopefully).
As to why they don't "fight againts Alduin and re-imprison him"; first off, why would they want to? Read Shor, son of Shor. They're just as much on his side as on the other.
Secondly, I don't think he's really "imprisoned", per se.
Thats the best way I'd be able to put it. I interpret the earthbones as the stabalized Mundus and the physics that are bound to it, and vice versa.
@ Sameron, regarding the Divines stopping Alduin, I'm sorry I don't know why exactly, but I don't think that it could work like that. Its not their job/obligation to stop him.
It is only one persons destiny to stop Alduin, as I'm sure we will all find out firsthand.