The mechanics for a warrior tend to fairly simple:- you run forward and hack & slash. Whatever build of warrior you get, they generally end up as a tank.
And in a party-based CRPG (e.g. Baldur's Gate) you are going to easily find party members who can fill that tanking role, because of its simplicity. In Baldur's Gate you have several npcs who make great tanks, especially Minsc, who is pretty much as good a tank as your PC is ever going to be.
Making your PC something other than a tank allows you to fill other roles, for example; Dynaheir is a great mage, but because of her specialization she can't cast Confusion or Chaos spells, and there is a serious lack of other good-aligned mages in the game. Your PC can fill that gap.
A varied party is an effective party.
Throw in the possibilities of game-over if the PC is killed (which is more likely on the front line), and playing a character other-than-a-warrior seems attractive. Not that it's not fun sometimes.