People make it out to football games more though because there are only 8 of them at home, and only 16 televised. Most of them come on during Sunday, when a lot of the country is off work. Football is just set up perfectly to where there will always be high attendance and ratings.
I think merchandise is just a good way of showing how much support a team has. The most popular player in the NBA usually has the highest jersey sales, so it would make sense that the most popular sports team in a certain city would have the most sold jerseys/shirts/etc. Also, most sports merchandise is pretty evenly priced. It cost about the same to get any sport's team t-shirt, and basketball and football jersey's are similarly priced also.
So the Rockets are the most popular franchise? in the last decade?