Because its not a real Pittsburgh team. The USFL owns all the team and its purely a made for TV venture. They know attendance will be terrible so they house all the teams in Birmingham to cut down on travel/logistics. Apparently a few will play in Canton this year so they are branching out a bit. But it makes little sense for the Maulers to play at RMU or Highmark in front of 500 people. This is the football version of studio wrestling. If enough of a fan base is built, they'll move the teams into their markets but I doubt that's ever going to happen especially now with the XFL starting up again.
This is about where I say that a spring league needs to have an NCAA Home Team component. PA/WV team with players from Pitt, PSU, WVU, Marshall, Temple, etc. Ohio team, Florida team, etc. I'd watch that.