I figured Ashura (himself) was connected to the Juubi in some shape/form, and the juubi's chakra/power influenced Ashura and his descendants, which could be why the Senju (Ashura's descendants) have strong bodies as opposed to the Uchiha (Indra's descendants). As for why Hashirama has been the only one to use it, probably Hashirama's chakra grew more/bloomed more than any other descendant of Ashura's. This is all speculation though, since Ashura hasn't been told to do anything bar some chakra avatar. Although it's weird that Hashirama didn't use wood style when he was young.