Go AI has been developed for some time, and the match happens now because it has reached the level where it can beat top human players. If AlphaGo is far above human level, it just means it's creators waited until they were very certain of a win.
How clear is this? If this just comes from professional humans saying "I would have played differently, and I could have beaten Fan Hui by more points" - well, we've seen that humans aren't necessarily very good at judging AG's moves, and we know AG doesn't care how much it wins by.
http://www.nature.com/nature/journal/v529/n7587/fig_tab/natu...