One of a few reasons.  It's either cheaper to do it at Bradley, the arena isn't available for the whole tournament, or Mohegan Sun decided that they could make more money off of some other convention than the PBA ToC.

Of the lot, I'd guess it's cheaper for the PBA to do it at Bradley.  They don't have to set up the lanes until Sunday, meaning they don't have to rent the stadium for three or four days straight.  Not to mention that if there's a concert or something on Saturday (when they're not bowling anyway) they won't have to tear them down and set them back up.

SH