Interesting but hard to argue with since as Mean Joe used to say about Pittsburgh: "If it's Sunday and you ain't at Three Rivers, you're in the wroooong place."
USA Today wrote what qualifies as an in depth expose for them into the survey methodology.
Of course a lot of the fan support was built during the 70's when these three guys dominated.
Pittsburgh did pretty well overall. While the Steelers were #1, the Pens came in #20 and our beloved Buccos were 72nd, all out of 122 teams that make up the four major sports.