I thought it was that they got bored of Brian and wanted a whole new personality. For new adventures with Stewie and what not, that way they could do a wider variety of conflicts. Stuff they couldn't do with Brian because it might not have matched his character stanza. They CAN'T just change him, so they killed him and gave them a new dog.
Finally an answer that makes sense. Seth McFarland is the creator of family guy, and the voice of Brian. His death was a hoax. Seth is alive. These guys saying that the guy who did the voice for Brian died needs to read.