Injuries. Really weird ones too. Caught his foot in a hole on the practice field on year and lost a year with foot/ankle issues. Knee injuries his last two years. Freshman were ineligible at the time, so he only had three years to play.
I don't think you can understate how huge of a recruit he was. Waterloo East had the national record for consecutive games won. Bear Bryant was in Waterloo trying to recruit a black athlete to the South. He was one of, if not the, first black recruits for Alabama. Johnny Majors said that Moses was better than Tony Dorsett who he coached at Pitt. He still is, arguably, the best recruit the state ever had.
57 carries, 256 yds, no TDs is all he had.