I know this won't change your opinion but in case you're wondering, there are legitimate reasons for the attendance differences.

You have to remember that the Pirates hold the for the most consecutive losing seasons in all US sports history, only broken a few years ago. This was due to abhorrent management of the franchise by the owner (McClatchy) who used public funds to build the stadium with the promise to field a competitive team. Instead, he consistently, year after year, would trade any player who showed any talent as a way to make a quick buck. He could have afforded a competitive team, he just didn't want to. The Pirates were simply a way for him to make money, he didn't care about winning, and the fans knew it. He completely betrayed Pirates fans' trust and so nobody wanted to support them.

Many Pirates fans went their entire lives until recently without even seeing a winning fucking season, let alone playoff appearances. You can cry all you want about your century without a ring, but you at least got to see winning seasons and playoff appearances. It wasn't until 2006 when ownership changed that the team started to rebuild from fucking shambles. It was only recently that the fan base was able to start trusting the front office enough to care.

Add to that that this is a fan base with a much smaller population than the Cubs to draw from. Pittsburgh ranks 2nd to last in population size for all MLB cities. Even splitting your population with the Sox leaves you with up to 4.5x the population to draw from, many of whom live much closer to your stadium, and many more nationwide. Pittsburgh is more rural, fans are spread out, and the Pirates do not have nearly the national presence.

Additionally, we have a great hockey team in the playoffs, who also happened to have a home series during the Cubs games. Most people are watching the Pens, not a regular season baseball game, and especially not a regular season home game when traffic would be a nightmare thanks to the Pens.