The majority of people will watch a Europa League tie, invloving Southampton and Manchester United, because they are English sides. (Hopefully that starting point, is obvious to everyone.) The competition is of minimal interest, to most fans, outside of their own club, or at least outside of the clubs from their own country.
Because it is a group stage, the other match in the group, will be of some more interest to those fans, than other matches in other groups, which don't have English teams in them. If Manchester United happen to be 4-0 up with 10 minutes left, some fans will switch over.
Is there any match you think Judio that BT Sport should be showing, that they are not.