Is there a reason why Domain-specific Cleric spells all seem to stop at 5th level spells?

@mikemearls Is there a reason why Domain-specific Cleric spells all seem to stop at 5th level spells?

— JP Robichaud (@controlenthused) March 10, 2017

Too few slots at 6th+ to make the extra spells really worth it. #wotcstaff https://t.co/FWYcyiGCsr

— Mike Mearls (@mikemearls) March 10, 2017