Probably a combination of lower ratings, and the cast wanting to move on to other things. I've heard that Winter wants to move more into feature film writing.
It would be a shame if it was just about lower ratings, as the dip was not that huge, and and besides Game of Thrones, HBO's line up is a bit soft. Game and True Detective are the only things that look good on it, with the ending of Board.
Lots of stuff ending this year Mad Men, Sons of Anarchy, Boardwalk Empire, Treme, True Blood, etc....