This is what I think is the mold for a successful Ninja Warrior.
-A Ninja Warrior needs to be speedy, agile, and have a good sense of balance. This is mostly necessary for stage 1, and into stage 2. The short time limit means that you not only need to complete the course, but you need to fly through it without making mistakes and having to pause.
-They should be relatively short and light. Extra height is usually something that works against you. We've seen this play out with a lot of the larger American Ninja Warriors who made it to stage 3. The three people who've completed the course exemplify this. The whole stage is spent with your arms holding on to something above their head, needing to support their body weight. The only area where height makes a difference is the Warped Wall and Spider Climb, but those are some of the easiest obstacles.
-They should have extremely developed shoulders, biceps, triceps, forearms, back muscles, and GRIP. These factors don't play much of a role in the first 2 stages, but in stage 3, they are all that really matters. And especially in Stage 4.
-In addition to that muscular strength, they need substantial muscular endurance. Stage 3 takes a fairly long time to complete. The average weight-lifting sets during training only last about 20 seconds at the most. But Stage 3 looks like it can take up to 5 minutes to go through. That is where most would-be Ninja Warriors have failed. They likely have the capacity to complete each Stage 3 obstacle alone, but trying to do 8 of them in succession is too much for them.
These Ultimate Ninja Warrior All-Stars all fit the mold, and is the reason they were so successful.