You have to remember that a lot of their local fans have been priced out and at a club like theirs there will always be someone willing to take up the seat, a lot of their season tickets are passed around by executives, bankers, people at work etc. A lot of people inside the stadium are just there to be entertained, whereas at other, less successful clubs, more of their genuine support attend the games and get behind the team. Plus like you say there's the tourist aspect too. That's one of the drawbacks of being a successful club.
The structure of the stadium doesn't help either, it's far too big. It is very, very difficult to get any sort of atmosphere going with 76,000 people in the ground, bigger stadiums are not always better. If one section of the ground starts a song, the people on the other side aren't going to be able to hear because of how far away it is. You need to be able to hear each other otherwise different sections sing different songs, out of time and it goes quiet again.
The manager himself has had a go at the fans, but when they try to make a bit of noise the stewards eject them and take their season tickets off them. The club don't want your local lad from Urmston or Stretford in there, they want the family who will sit down, shut up and spend money in the megastore.