I actually have thought about this recently as well. He trained for a very short time on Dagobah with Yoda, and then took off to Bespin where he got his ass hammered by Vader. He then spent what, a couple years, couple months, between then and the events of Return of the Jedi? There's a lot to be said about practice and whatnot, but I guess his real growth seemed to come after ROTJ, while the New Republic was forming. It is an interesting question, and I have to wonder if Luke didn't learn from the mistakes made by the Jedi Order during the Clone Wars. Being the last of the Jedi, beginning everything anew, it only made sense to try something different.