According to Lucas, he was exhausted and thought it would be easier to manage that way, I believe he gave himself a heart attack making ANH (literally). But then when TPM came out he said he would direct all the prequels because getting another director just meant another person to fight with over creative differences and he wanted to elimiante that as much as possible. A purer path of getting his ideas to the screen with fewer intermediaries is how I interpretted it.

Whether this was the reasons back in 1980 I do not know. GL has a habit of changing more then his movies with time.