Growing up in Manchester I'd say it was pretty 50/50 Utd and City fans even in the 90s when Utd were on top of the world and City were in the 3rd division. Utd obviously have more national support but in Manchester itself it's pretty even in my experience. Not sure why people weren't at the game. Maybe just because they knew it was going to be like a training match and pretty boring to watch? I dunno.
Teams kept deciding that a cup match against Burton was a good chance to rotate and promptly lost, combined with some lucky draws (toughest team they faced was Burnley, who were struggling at the start of the season and had had to play six Europa qualifiers).