The Walt Disney Company (Disney) bought Lucasfilm Ltd. including its subsidiaries in 2012 for roughly $4 billion USD.
Star Wars was a property owned by Lucasfilm and so is now under control of Disney Management. For more information, see this USA Today article and this Bloomberg article.
As for George Lucas not being in the Director's chair for Episodes V an VI, it seems to have been a personal decision for him.
"I dislike directing. I hate the constant dealing with volatile personalities. Directing is emotional frustration, anger, and tremendously hard work - seven days a week, twelve to sixteen hours a day. For years my wife would ask why we couldn't go out to dinner like other people. But I couldn't turn it off. Eventually, I realized that directing simply wasn't healthy for me."
- George Lucas: Interviews Edited by Sally Fine 1983
But, prior to the prequel trilogy he seemed to have changed his mind...
"It's not that I prefer producing, that's just where I find myself these days. It wasn't something I did by choice. I was just more effective in the producing area, which is where the opportunities have presented themselves in the last few years...Ultimately, I'm a director. I've done many things, but that's where my heart is...I just want to get back to what I started doing.
- George Lucas: Interviews Edited by Sally Fine 1983
Also, we have some good questions and answers over on SFF.SE that related to "who, what, when, where, why" of George Lucas' involvement going forward.