ability to play

Because they're an entirely different team. That would be like saying why do the Steelers win Super Bowls and the Pirates can't even get to a playoff game? Just because they're from the same area means nothing, other than perhaps a fan base. Cubs tend to put out less money for their players in general, have a totally different staff to train them and a whole different style of players even.

I am not a big baseball fan but I would say that it has to do with multiple factors. Maybe the White Sox are better with money so they can go out and get better players, plus maybe they have a better coaching staff. It could be that the players mess better as a team than the Cubs and it couls also be because of who they are playing, maybe the Cubs just play better teams than the White Sox. I think there is so much more that goes into building a good team than just getting good players. You could have a whole roster of mediocre players and still win lots of games if the coaches are good, the team plays well together and if the players have heart.