I believe on Occam's Razor theory, which basically states that the right answer is usually the simple answer. With Rose's injury, it became a need to see development from Teague. He is only 20 and could still be 2 years away from substantial playing time.
Sending him down to the D league does just that. It gives him full time on the court so he can get stronger and develop. As much as the Bulls want to believe that Rose will come back strong, they have to starting seeing who can replace him if needed.
This is the simple reason he was sent down, to get more playing time so the Bulls will know what they have if they have to look for a replacement for Rose in the future. He was brought back because James got hurt, that's it.