The team was what 5 games under .500 and buried in the WC standings at the break? I think there are a couple of things at play:
-Late blooming team so advance ticket sales aren't as strong
-Dynamic pricing designed to optimize revenue more than optimize attendance
-The Cubs simply aren't the same franchise that could open their ticket booths in mid-Feburary and expect half the home games to be sold out by the time the season starts. Pre-2003, this is basically what Cubs attendance looked like. 2003 brought a surge of fans desperate to see the Cubs end their WS drought and now that they have, attendance is going to be a little more elastic.
Their attendance floor is still pretty damn high, I only see 1-2 games after April that were below 30k in attendance and they were just below 30k.
Maybe that doesn't explain everything, but just some thoughts. Personally, I'd probably go to 5-6 additional Cubs games per season if the ticket prices were more like they were in the 00's and early 10's. But if it is going to cost me $300 minimum to take wife and three kids to the game, I'm only good for a couple games a year.