Warrior is only top pick when you look at total, most of which are tanks, not DPS. DPS Warriors are actually quite few, as most people go Rogue instead for melee. Comparably, Rogue was most played DPS, then Mage, Warlock, with Hunter buried down at the bottom next to DPS Warriors. There's a few reasons why. Firstly is probably because the tier sets are all for tanking, forcing you to fill up with non set DPS gear for your entire raiding career. Then you have the fact that Rogues can control their threat; a Warrior cannot.
In Classic, most classes were intended to raid with 1 build. The others were not supported. For any healing class, it is the healing tree. With Warriors, it is tanking.