The main problem in incorporating the Rally3 cars is that they are slower than Rally2 but are intended for the same championship. Everytime a championships goes from faster to slower vehicles it's very problematic hence why it happens very rarely. When S2000 came they were faster than Gr.N (at least on asphalt at the beginning). When R5 came they were faster than S2000 (ar at least similar at the beginning). When Rally4 came they were faster than R2. In all cases switching to them was no big issue. The new cars were simply better and teams switched to them naturally. Now it's different. The change of cars in the very same championship to slower ones mean that all the faster cars which the involved teams already have are useless. In essence it's not much different to fifteen years ago when many national championships banned WRC cars which was a step which is very controversial till this very day.