Way back in the old forum days when devs actually dared to visit the forums and post/discuss things with the community, there was mention made of the difficulties with the game engine in getting character avatar animations to have multiple appendages that were functional and controllable. At the time it was in response to a thread asking about the Xindi insectoids being added into the game.
But, we have since had the "multi-legged race from fluidic space" and the Tholians added so I don't know if the issues are still insurmountable.
There are less limitations on NPC models as they're controlled in house with zero customisation on the part of the players.
If they gave us a playable four legged/armed federation race to use, not only would they have to double the current uniform options to map them to the extra appendages properly (seriously look at the texture mapping for clothes in all the current races, it's exactly the same from race to race currently) and redo all attack and weapon animations to take the extra limbs into account.
Conversely with an NPC race they'll normally only use one weapon and a set series of melee attacks (if any at all) and have their "uniform" designs baked into their model. A lot less work.