Why should rules be added just for the sake of it?

Firstly, a 4wd chassis is only going to have an advantage when you have more power than grip, so that counts out 10.5 or higher wind motor classes and considering even mod 1/12th on carpet has so much grip that grip rolling is more of a problem than putting the power down. The only time a 4wd 1/12th would have a chance of being competitive is in mod on asphalt, so if a manufacturer was looking at one it would be a one off special for Florida and not something they would put into production. I'm sure if someone turned up at next years worlds with a home made 4wd chassis that happened to do well there would be more calls to guarantee 1/12th runs on carpet in future than to ban 4wd.

Secondly, a 4wd chassis has to fit under a homologated bodyshell, so the taller Tamiya Mazda above wouldn't be allowed. You would either have to build the 4wd drivetrain around a chassis layout with the motor in the same location as in a pan car, or right in the centre with a shaft drive so the motor can fit under the cockpit. Even the old 4wd 1/12ths wouldn't fit under a modern body as the sidepods on a modern body are way too low for the chain and belt drives they used. Your only other alternative would be to submit a custom made body for homologation, which includes having to make them in sufficient quantities to prove they are commercially available, even though you will never sell any of them.

Personally I did actually work on a 1/12th design based on a 4wd mini chassis a few years ago to see if it was actually viable just as a technical challenge and struggled come up with something that fit under a body. Today the bodies are even lower and I would be mightily impressed if someone can build a competitive 4wd drivetrain under an AMR12.