One problem is that (AFAIK) they don't have fare machines in the bus center, only down by the station. No problem for a local with a card already (either Chicago card or a transit cards), but for out-of-towners, they'd have to send them into the basement to the station to get a card, then back up to the bus. They could charge on the bus in cash, but then they'd have to issue a transfer.

The CTA probably figures that the number of people using it without getting on the L at Rosemount is small enough to not be worth the cost and hassle of dealing with fares at ORD, especially when it was only 3 weeks.