The X-patterns are a really good tire, offered in the same compound as USVTA racers have been using for 3-4 years, and a slightly softer "pro" compound. They are 26mm wide, which fit a myriad wheels in offsets that allow for both 190mm and 200mm bodies, and they are in plentiful supply. If they aren't and they are used by everyone, HPI will ramp up production just like they did for VTA vintage tires.
These are the kinds of simple structures and rules that would head off the racer's inherent tendencies to look for that last little advantage before they ever hit the track—and cause people to spend more than they normally would if everyone was using the same thing.