When WoW first began, buffs were a filler for DPS. A paladin would deal less damage in order to up the damage of fellow raiders. Obviously, when WoW's paladin was first introduced, it was designed to play much like Diablo 2's paladin. The main difference: it didn't work. Paladins, the premier hybrid class, through trial and error, has become an equal threat in the DPS meter as your staple DPS classes, like mage and hunter. Thus, there has been constant strife over "pure" and "hybrid" DPS.
Why didn't it work? One explanation can be that World of Warcraft is filled with narrow-minded nitwits (might be plausible). The people who don't even have the sense to factor in the benefits of buffs just take with them a warrior or a rogue. Another, more realistic hypothesis is that WoW is more PvP oriented than Diablo 2. Diablo 2's PvP function was strictly limited to dueling, whereas many can state that PvP in WoW is half the game. A buff heavy class would have to dry hump and bend over for their allies is no fun. And to the paladin's defense, it really was no fun.
However, the third explanation I have is much more tangible. A lack of a sophisticated DPS meter showing the contribution of buffs. Blizzard didn't implement a DPS meter when the game was first released and so the players had to use a third part add-on. Recount came after DPS Meter, which was very primitive at the time and only showed the individual damage output of each character. This quickly became a standard for raiding and so paladins were pushed aside. In response, the paladins kicked and screamed through several patches and a whole expansion before they were cast into the league of competitive raiding. Imagine how different the paladin class, as well as the shaman class, paladin's twin brother, would've been if Blizzard had just thought of adding a dedicated DPS meter into the game.