In some countries (Finland, Poland) only Protos with Evo or Subaru parts allowed. Plus I don't think any modern 4wd system will be really useful in proper rally car. That's also why N4 died, as not anymore any suitable cars for it...
I know properly rebuild evo parts are not bad, but then you need to compare prices with used R5 cars, not with new ones. And still for example used Sadev/Xtrac/Ricardo rear diff will be much better condition than used RS evo diff with basically not known mileage... And this is not a big problem on gravel, but with powerful cars on tarmac RS rear diffs for example are weak and not easy to get good condition one.