I find it odd that the directors of Star Wars Episodes V and VI remained in relative obscurity despite having conducted such relevant films. Why did this happen? What was exactly their role? And why didn't George Lucas direct the movies himself?

I am not quite sure of your definitions of obscurity. In the case of Irvin Kershner, the director of The Empire Strikes Back, it simply isn't the case at all. He went on to make a number of other reasonable high-profile films including the James Bond film Never Say Never Again (1983) and Robocop 2 (1990), among others.

Richard Marquand, the director of Return of the Jedi, made a couple of other films but the probable reason for his obscurity is that he died of a stroke only a few years after completing Jedi.