Alexander is correctly noted for being weak on the ground but he also has significant cardio problems. Part of his problem against Schafer was that he was gassed after only two minutes (as well as being a fish out of water on the ground).
He's overmuscled. You can't have that much muscle and expect to fight for more than 2-3 minutes at any kind of pace. Your lungs just aren't big enough to supply that much muscle with oxygen. Look at all the top fighters and while almost all of them are in good shape, none of them have that insanely "overdeveloped" weightlifter build that Alexander has.