The difference is that Mars Direct was a conservative plan, wrt rocket technology. It just used existing Shuttle parts and was meant to fly from Shuttle pads in between Shuttle flights. It didn't require developing anything new, that's why they were sure it would be cheap and fast, and flight rate would make it possible for it to be reliable. SLS is that idea turned into a jobs program.

Regulations are hard enough just using methane, I wouldn't want to be the one filling out the paperwork if that was a nuclear rocket.