There was a blue post awhile back explaining it (I'm pretty sure anyways) that explains why they cannot become monks and it makes sense. When you (the player) make a worgen or goblin, you are starting off technically in the past. At this point in time, there are no monks before the cataclysm. So, they cannot be monks sense at that point in time the player worgen/goblin would have already have chosen their class. The only way they could logically fix this is by making goblin/worgen monks skip their starting zones and have a special starting area as monks after the cataclysm, which seems like a lot of effort built around two individual races.
By that logic, there shouldn't be Blood Elf or Draenei monks, either. Their starting zones are in the past, too. If you start a Draenei toon, they still talk about the Exodar crash like it just happened.