The catchment argument might hold more water (heh!) if they could point to other examples.
There are so many other factors involved: history, current success, past success, quality and accessibility of the stadia, class identification, etc.
I live here in the San Francisco Bay Area and the San Francisco-based clubs (baseball, American football) have always had larger fan bases than those based in Oakland. There have been periods were the Oakland teams were much more successful, but the San Francisco teams were always more popular. Oakland has a larger "catchment area": it doesn't mean shit.