A bunch of reasons, or so I read. They wanted to make a big change to shake things up and mess with the boring rut the show's been through, and killing anyone else from the main cast would've been too much. Brian was their sacrificial lamb... dog, but it's Family Guy, so they can always bring him back when they want to.
WHAT? THEY KILLED BRIAN? NOOOOOOOOOO. WHY NOT CHRIS?
Lack of viewers. He'll be back once they rake in enough millions. Should have been Meg though....
Im sure he isnt gone completely. Just like Cleveland, he will have random appearances every now and then