RG3 and fellow rookie Morris took the league by storm with the read-option under Callahan in 2012. He was/is a running QB but wasn't built to take the hits at the NFL level. To top it off, it only took the league one year to stomp out the R/O just like they did the short-lived Wildcat offense. Bottom line, RG3 wasn't equipped (largely by his own ego/sense of entitlement) to evolve into a pocket passer.
Dak, on the other hand, is what you call a "reluctant runner". Unless he's absolutely pressured, or by design, he will stay in the pocket. I don't know what Dak's ceiling is, but I believe we are seeing his floor right now and, at a minimum, can be an Alex Smith-type bus driver for a decent amount of years.