Mostly because would be far too time consuming to make sure every setup in the venue has the proper CaS formula. Most tournaments have console setups that are brought by attendees themselves because organizers simply can't provide enough on their own, and each of those setups would have to be checked to make sure that they have a DJ that's tournament legal.
Not to mention that after checking every single setup (which *would* add a ton of prep time for organizers) there are other issues to consider. What if you discover that one console's CaS is slightly off half-way through the tournament? AFAIK you can't go back and check/change a CaS height once you've saved it, so basically you'd have to either eyeball pre-made CaS or create them at the venue before the tournament starts, which would cause huge delays.
Even small local tourneys organized by experienced people take a long time to set up without having to deal with CaS. It's better to just ban them outright than create a whole new set of regulations that are particularly susceptible to abuse just to use one character.
As for online tourneys allowing CaS, well, I guess that would be up to whoever decides to run/organize things. But given that you can't check people's CaS online in the way you can offline, I'm pretty skeptical that everyone would adhere to the 'standard', whatever it ends up being. Personally I wouldn't be confident in looking at a CaS and saying "Oh, that one's DEFINITELY not the default size/build." I'm not sure anyone would be able to do that. And then, what if someone using DJ *is* accused of not using the default settings? Again, I'm not sure there's a way to check that fairly.
