when all you have is a spear, going after "slow", grazing animals is "easy", but the Charismatic megafauna that survives today is in general, the ones you don't mess with (again, when all you have is a spear). Also, moose, bovine, etc... don't have hunting instincts like the big cats, or weigh 2,000lbs like hippos and rhinos... there isn't as much cover on the African plains as there is in the US to go after the larger, horned animals.