In 1991, just like there will be this year, plenty of locals come out the woodwork to support the team in a national cup final. There's nothing wrong with that.
Plenty of people who aren't really football fans get caught up in the excitement of it all and are happy enough to come to the game with their Motherwell-supporting son, daughter, husband, uncle or whatever.
As for there being double the fans in 1991, it may simply be a case of more people having less to occupy their lives back then.