The ranking is a combo of "Fan Equity," "Social Equity" and "Road Equity."
The Pats come in #3 in "Fan Equity," which is defined by people coming to games and opening their wallets. However, #1 and #2 there are DAL and SF, each of which has built shiny new stadiums, so coming in third behind teams with those advantages ain't bad.
The Pats are #1 in "Social Equity>" I suspect that this Board can take a bit of the credit for that. Plus, players like Brady and Gronk have massive Social Media followings.
The Pats are #5 in "Road Equity." If I look at the teams ahead of the Pats, #1 is Dallas. Here, I'm guessing that there is a confirmation bias in that the Cowboys are often called "Americas Team." #2 is the Eagles, which had me scratching my head, until I reflected that Philly is about halfway between Washington and New York, homes of their Division Rivals, so it's relatively easy for their fans to get to two big Away games. #3 is the Raiders, so here again there might be a confirmation bias going on because of all the talk of "Raiders Nation." #4 is the Giants, who are also relatively close to Philly and Washington. But the big factor here might be the ease of travel from the NY area to anywhere in the country from three different Airports and the relative affluence of their fanbase.