#3 Roman Reigns has never won the King of the Ring tournament
Throughout WWE history, the King of the Ring tournament has been used to crown up-and-coming Superstars to shine the spotlight on them. Stone Cold Steve Austin rose to superstardom after winning it back in 1996. Brock Lesnar too sent shockwaves through the pro wrestling industry after reigning supreme in the 2002 edition of the tournament. Roman Reigns never had this opportunity though.
Unfortunately for The Big Dog, King of the Ring had become a sporadic affair by the time he was starting to find his ground in WWE. In fact, only three KOTR tournaments were held in the last decade. Sheamus won it in 2010, Bad News Barrett was the victor in 2015, and most recently Baron Corbin claimed the throne in 2019.
It's unlikely that Roman Reigns will ever become the King of the Ring given that he's the poster boy of WWE already. But as far as Vince McMahon is concerned, you never say never, so one can still hope.