Actually it could be passed in the senate... So many states have medical and adult use marijuana now, that senators within those states may be forced to reconsider their positions, with the upcoming mid-terms... They won't want their opponents using their anti-MMJ stance against them. They only need a few republicans to swing to their side on this issue...
There is no chance it would be passed as a stand alone bill, that is why it was added to the NDAA, it is a must pass bill that Senators likely won't hold up due to the SAFE Banking provision.