Kevin Arnott's right boot
Striker
The flippant answer is "because it was in the script".
The answer given by Ian La Frenais is that the scripts were written by different people and at different times. "The Girls They Left Behind" was one of the first to be written and, at that point they hadn't decided whether to make them Sunderland fans or Newcastle. Sunderland were in the First Division at the time with the Mags in the second. Particularly as it was pre-Keegan, it probably made more sense that Sunderland had a European game.
This doesn't, of course, explain why they didn't subsequently change the scripts to keep them as Sunderland fans in other episodes, or to change the Liege game to a Newcastle match but, I suppose you have to take into account that La Frenais had lived away from the North East for many years at this stage, and would have grown up at a time when going to watch both clubs was a perfectly normal thing to do, so perhaps he thought that this hadn't changed.
The answer given by Ian La Frenais is that the scripts were written by different people and at different times. "The Girls They Left Behind" was one of the first to be written and, at that point they hadn't decided whether to make them Sunderland fans or Newcastle. Sunderland were in the First Division at the time with the Mags in the second. Particularly as it was pre-Keegan, it probably made more sense that Sunderland had a European game.
This doesn't, of course, explain why they didn't subsequently change the scripts to keep them as Sunderland fans in other episodes, or to change the Liege game to a Newcastle match but, I suppose you have to take into account that La Frenais had lived away from the North East for many years at this stage, and would have grown up at a time when going to watch both clubs was a perfectly normal thing to do, so perhaps he thought that this hadn't changed.