Or it could be that they think they've done enough homogenization and don't want to go any further by adding a 3rd/4th usable/available anytime spec. It wouldn't really surprise me to hear that they think they've done a little too much already to be honest. They put something in as an experiment. It got datamined. Internally, after playing with it a little bit they didn't much like it. So it's not going to happen.
It's possible for it to be really simple.