I'd say it's not nearly that simple.
History - New England has a history of being really good, for the last 15ish years, the Bills, not so much
Starpower - Brady/Belichick are big names, and big names are easy picks
Laziness - We all know a great many "experts" basically only pay attention to the big market teams, because that's what they are paid to do.
I bet all these contribute, personally.