To be fair, the reason that more of their games are shown along with Chelsea's, City's etc is because as the season draws to a close, the focus shifts to the title race and the relegation battle. These games are infinitely more exciting to the neutral and the Premier Leagues global audience, as opposed to a game between Southampton and West Ham, neither of which have anything to fight for.
I don't think it Is anything to do with bias, the fact is they are an entertaining team to watch , and invariably score lots of goals. These are the types of games Sky want to be showing as opposed to the likes of Newcastle and Villa last Sunday, which was absolutely atrocious to watch, but they do have a minimal quota to fill for each club.
If we were involved in a title race or a relegation battle, we would be on the telly an awful lot more than we are. United are probably in more than us because they are the Champions and everyone is incredibly intrigued by their plummet down the table.