Why didn't Lelouch go to his father as soon as he got his Geass? Is there any reason why Lelouch didn't go to his father to find out the truth about his mother's death as soon as he got Geass?
You can't just walk into the throne room of the Britannian emperor. Bear in mind that Lelouch was no prince anymore in title, he was banished (or something like that, I can't recall the exact term). It's like walking in the room of Queen Elizabeth or the POTUS; it's not something you can physically do. Even with Geass, he'd just be shot down by a sniper or Knightmare.