Making the race model is easy. Adapting all the armor for it is the hard part.
Up until MoP, all armor except helms and shoulderpads were just painted on the Race’s model (even capes, the cape is part of the model, that part just stays invisible when you aren’t wearing a cape).
So when Blizzo added Blood Elves and Draenei, all they had to do was to adapt all Vanilla helmets and shoulders from vanilla to the new model.
When they added Goblins and Worgen, they had to adapt all helms and shoulderpads from vanilla, tBC and WotLK. A lot more work but still doable.
When they added pandas, they had to adapt all helms and shoulderpads from vanilla, tBC, WotLK and Cata. That was the last “easy Race”
MoP introduced the concept of 3D belts, and Legion exploded the concept with 3D everything.
When BfA came and the Kultirans were created, they had to adapt every helm and shoulderpad from tBC, WotLK, Cata, MoP, WoD and Legion, plus the 3D belts from MoP, WoD and Legion, plus the 3D chespieces, pants, gloves and boots from Legion.
As you can see, as time passes and the game adds more and more gear, creating a new Race model becomes more and more unreasonable.
If they were to create a brand new Race model in The Last Titan, they would have to adapt all the helms and shoulderpads from tBC, WotLK, Cata, MoP, WoD, Legion, BfA, Shadowlands, Dragonflight, TWW and Mid, plus all the 3D belts from MoP, WoD, Legion, BfA, Shadowlands, Dragonflight, TWW and Mid, plus all the 3D chespieces, pants, gloves and boots from Legion, BfA, Shadowlands, Dragonflight, TWW and Mid.
I could be wrong but it seems to be that the humongous backlog of armor made the sheer amount of work necessary to create a brand new Race just not worth the return of the investment.