A lot of it had to do with the curse of the billy goat. There was a feeling at the time among Cubs fans that the Cubs were cursed and could not win. Bartman was just another event in the negative mindset reinforcing the myth. Now even more people believe it.
In 2003 (incidentally, the Chinese zodiac's Year of the Goat), a group of Cubs fans headed to Houston with a billy goat named "Virgil Homer" and attempted to gain entrance to Minute Maid Park, home of the Astros, division rivals of the Cubs at the time. After they were denied entrance, they unfurled a scroll, read a verse and proclaimed they were "reversing the curse." The Cubs won the division that year and then came within five outs of playing in the World Series, but were undone by the Florida Marlins' eight-run rally immediately following the Steve Bartman incident. The Cubs then lost the following game and with it the series. (The Marlins went on to win the World Series against the New York Yankees). Further salting the wound, the Astros earned their first World Series berth two years later (they would be swept by the Cubs' crosstown rivals, the Chicago White Sox).
Younger Gamecock fans may not recall, but there was once a belief amongst Gamecock fans that we were cursed by the notorious racist, Pitchfork Ben Tillman, a man who has a tower erected at Clemson in his honor. The curse ended when Brad Scott took it with him to Clemson. Now they have the Tillman curse.