Honestly, it's all about balance. Games like Dungeons and Dragons are generally built from the ground up to support multiclassing. Each class has its synergies, and multiclassing its own rules, and there's a limited frame in which they work which works with the whole concept of multiclassing. With WoW, you have 10 separate classes so minutely balanced that even the slightest outside influence can shift the balance of power in class representation and role. With millions of players, each of them expecting to be able to fulfill a certain role and be competitive with everyone else, and each class balanced around a very specific set of skills and equipment, adding multiclassing to WoW would require such an extensive redesign that it would make Patch 4.0 look like a hot fix.