That's what Kellen Moore was known for- his ability to judge the receiver's rout and throw on time.
So it's not a surprise that Cooper is better suited for it, having spent so much time with Moore and practicing.
Jason Garrett taught Dak, and was no one to be teaching. Jason was so desperate to believe Dak would finally justify his existence that he coddled Dak and didn't make him play under pressure in preseason, and Dak learned to get by behind what was probably the best offensive line in NFL history, coupled with a young Ezekiel Elliott distracting defenses.
So Dak didn't learn the tactics/mechanics of professional quarterbacks.