Because the GOPe will stop at nothing to take Trump out and get a GOPe candidate in. Until the campaigns official suspend themselves from the race, he needs to keep hitting...all of them.
NOBODY likes Jeb Bush. Republicans do not like him. Conservatives do not like him. Democrats and liberals do not like him. Donald Trump is playing his little game. His goal is to get more voters over to Hillary Clinton. Trump is in it so Hillary can win it.