Because Warrior is the best Pick in Arena.....
There are way too many mages in Arena for warrior to be good. Paladin and Mages make 50% of the 7+ wins population.
You know if you face a Druid, Shaman, Warlock, Rogue or Priest or Hunter in Arena you must expect strong combos.
A Warrior at such high wins means his deck must be the real deal.