Because Jan-May was the real "summer season" with Civil War and Zootopia grossing a billion dollars, Jungle Book coming close, and Deadpool, Super V Batman, The Mermaid, Kung Fu Panda 3 all making over half a billion dollars.
http://www.boxofficemojo.com/yearly/...yr=2016&p=.htm
I think quality has been a big factor unless you're a DC fan the grownups have become more discerning. I think the best place to look at that is the horror genre.
Conjuring 2 - 300+
Lights Out - 140+
The Purge 3 - 110
Shallows - 110
10 Cloverfield Lane - 100
Don't Breathe - 92
The Boy - 60
The Witch - 40