Because even after the initial Order, the Jedi were still hunted down even if they weren't in the battle? I ain't saying that some Jedi couldn't survive, but we don't need a whole lot or some just appearing from thin air. There have already been 4 jedi that survived long enough, to join Luke's order..which is fine, but I don't think we need more then that.
Did Kenobi live years on Tatooine after 66? Yes. Therefore it's possible other force users especially those not in the Order and not in the initial attack survived. Plus there was Yoda self-exiled on a planet too.
And you sure of the 4 again? Did Disney already say the rest of the cast of the movie? I missed that. Link please? If you are talking about the books, they may or may not be in 7. Only force users Disney has even official said are Luke, Leia, and their kids. We don't even know if it will be the same kids.