He pitched as well as they could have expected when they gave him this contract, so I suspect this has more to do with playing time than money. Yes you can never have too many starters, but if they think they're going to have 5-6 other guys in the starting rotation to start the season, there's no reason to pay Rea to be seventh starter.
Peralta/Woodruff/Myers/Civale are probably locks, there are a couple internal youth candidates to claim a rotation spot, and they may be looking to add a better starter than Rea through free agency.