The TRD wheel has a different offset that widens the vehicle's overall track by about an inch. Because of this, they most likely chose a narrower wheel to minimize the chances of rubbing. Shortly after the TRDPs release in 2015, Toyota issued a TSB and started shaving their fenders at the port due to some having clearance issues.
T-SB-0009-15 TRD Pro Bumper/Fender Interference - Toyota 4Runner Forum - Largest 4Runner Forum
EDIT: Also, the TRD wheels weren't intended to be exclusive to the TRD Pro. First and foremost, they were designed as an OEM accessory that needs to fit all trims with no additional modifications required.