First off, you're always going to hear more from the vocal opponents than anyone else.
Personally, I think anyone whose told they're "brilliant, a genius, a visionary" long enough lets it slowly go to their heads. So much so that they begin to believe their own shit doesn't stink. In Hollywood especially, people tend to shelter and not confront or question or challenge these people. When frankly, it's being questioned and challenged and confronted that made them good in the first place. Lacking that sounding board, lacking that challenge, they devolve into mediocre hacks.
Lucas, Roddenberry, Abrams... people of their ilk: they make good stuff initially. Then they make GREAT stuff. And when they've done that greatness, they start to think they've got the midas touch. And no one stops them and says "hey, this sucks, fix it" because... I dunno why. I don't know.
I think the reason Lucas gets so much more vocal hate than almost anyone else though is that... he really did change things. He really fought the good fight and beat the studios and the corporations at their own game and made a mint doing it. He came out of no where, changed things radically, and came out on top and was a lot of people's hero for it.
So when he suffered the same fate as the others, the effect just felt worse. Because what he had done had been so much bigger.
There are those who lament Roddenberry's flops and failures. There are certainly those who (validly, IMHO) critique Abrams for his many failings. But with Lucas... it feels more personal to so many people for various reasons. So the level of vitriol is proportionate to that, imho.
Look at any creative individual though, especially modern times, and you'll find the same thing. They rise, they become hailed as the greatest thing since sliced bread, and eventually they lose that spark and become more mediocre. It happens to everyone. Scorsese, Spielberg, Lucas, Roddenberry, Abrams, Woody Allen, you name it.
Creativity is everywhere. Channeling that creativity and making a good end-product from it takes pressure and challenge and drama and luck. Which is what a lot of the greats had at their heights. Once they've achieved success they didn't have to fight anymore, and without that fight and that challenge and the doubt and questioning, the pressure is off and the product is worse. *Shrug*