Because the REDS are going to have to medically study his arm over a period of months while he has a series of starts in regular season games.
They can't do this at the Major Leauge level as his playing time would hinge on situation and Dusty Baker, along with the fact that they'd find it hard to control what he throws.
The REDS have very little information on Chapman's "exact" past as far as innings pitched, what he threw in those starts, etc., etc..
The REDS are going to have to have him throw in the minors under "controlled conditions"; and, then they'll peform ongoing medical tests to see how his arm reacts.
Only after they have this information can they come up with a "plan for his future that includes letting him loose in the Majors".
Any plan other than this is not prudent, and they didn't invest all of that money so that he can throw one season and then have an injury.
While he's in the minors, they are also going to tweek his mechanics so that his arm lasts through the life of the contract, and so he can get to 210-220 innings on a regular basis without fear of harm to the arm.
The REDS are going to be very, very careful with this "child".
It's practically like adopting a kid from Africa. You're going to spend the first year doing all types of adjustments both medically and physically to the child.