Personally, I think there were people that started with old wc2 abilities and units from the beginning of the developing of Warcraft 3(as a part of rpg like Warcraft universe mix of tactics and strategy). Also, as they were need some props for Medivh's memories, that Blizzard even quotes in their trailers and demos, considering latest playwarcraft3.com changes.
So the
main reason is basically this, time 1:26: