A big part of it is the "I'm not feeling it". I hope I'm wrong, I loved the first and would really enjoy seeing this summer have a breakout. But you are right that just because nothing so far has broken out doesn't mean nothing will.
I will put it this way, if any two movies this summer are going to make a run at $300 million, it's Dragon 2 and TF4. It's just for some reason the closer we get to the dates for both I just feel less and less like they will do it.