There are no reasons. That's just not obvious to you.
If they wanted Cavan Biggio in AAA, Cavan Biggio would be in AAA.
Rehab stints are often used to help someone recover their form after an injury has robbed them of a decent amount of time for the same reason that people are sent to extended spring training if they miss time during spring training. "Recovered from injury" doesn't just mean "doctor has cleared them to play", it means they are ready to go in MLB games.
He played a total of 22 AAA games during his rehab stints, and he was not hitting well when they promoted him, so why would they have promoted him if they were going to "wait until they start hitting" [citation needed on that quote, btw.]