Why? Here's a couple of reasons;
1.
Armor. Add one more class that wears mail armor, and every armor type can be used by three classes. With the new WoD armor changes, this has become even more apparent.
2.
No more Heroes. WoW classes are derived from WC3 heroes and units. There's only a couple left that haven't been utilized by existing classes, and those remaining heroes and units all share a similar theme.
3.
Lack of Design space. Existing classes are already showing some overlap. There simply isn't much design space left to derive a whole lot of new classes out of without things becoming redundant.
4.
Class types. The game has roughly three class types, and it's entirely based on the roles a class can perform. There's pure DPS, or single role classes. There's hybrid, or two role classes. Finally there's true hybrids, or three role classes. Pure DPS and two-role hybrids have four classes each. Three-role hybrids only have three classes (Druids, Paladins, and Monks). If the next class is a three role hybrid, there's not much need to create another class.
5.
The last open space. Okay, this may seem like a stretch, but if you check out the WoW official forums and check out the class forums, you notice that there's space for one more class. If you go over to Wowhead and check out the WoW talent trees, you'll also see that there seems to be only space for one more class. It would appear that adding that 12th class would bring balance to the world, as well as the class lineup.
Agree or Disagree? Let me know what you think.