How many people know Seth MacFarlane? How many people know him as Peter Griffin from Family Guy? I'd say his name alone isn't enough of a draw, not for a Hollywood movie, so that might be why they went with a similar voice, for recognition value. Because, yeah, that bear is totally Peter.
Mind you, no one has commented on Wahlberg yet. I mean, look at that guy! He's a somewhat respectable actor now!
(we're on Tumblr too!)
Someone on the Bendis boards was wondering how people would react to the trailer if they didn't know who McFarlane was or have a preformed opinion about his other projects. I'm kind of wondering that myself; I know my own response to the trailer is at least partly influenced by my prior knowledge/opinion of the dude/his work, so I'm curious how I'd feel about it without that.
Hate me all you want, 95 % of the time I'm write.