There's no question that Apple could have modded the Plus one more time to improve SCSI performance and compliance. That they chose not to is very telling, and H3NRY's supposition is likely spot-on: Apple probably didn't want to cut into the SE's sales. The SE was actually cheaper to build than the Plus (fewer manual assembly steps associated with the analog board). They would have wanted to encourage a transition away from the Plus as quickly as possible.