#4 He didn't need titles to fuel his success
It's an unwritten rule of professional wrestling. In order to be considered great, you need to win championships. You rarely have legendary wrestlers who haven't won a number of World Championships. If you don't win a huge number of titles, you just aren't that good.
Not for The Undertaker, though. "The Deadman" did win his fair share of World Titles but he was never up there with the Ric Flairs, John Cenas, and Triple Hs of the industry. He just didn't need them to feel like a big deal.