All because Dooku can do it doesn't mean Maul can do it. Maul isn't all that powerful, yes he was trained by Sidious, but maybe Maul doesn't have enough Midichlorians(SP?) to generate such power, he's a great swordsman better then Dooku but Dooku is far more powerful then Maul. Also I believe Vader can do it GL is just making up an excuse why he doesn't do it b/c its too late to change it...remember GL made up OT has he was going along, he probably just thought about force lightning when he was making EP VI so can show how powerful the Emperor is