#1 Can't afford to lose him

Whether the WWE Universe wants to admit it or not, there was a time when Seth Rollins was literally the biggest Superstar on Monday Night Raw. Not only was that evident by how he conquered 'The Beast Incarnate' and won the Universal Title from at WrestleMania 35, but also by his dominant run after that.
With that being said and a lot of fans still not willing to turn on Rollins just yet, it makes no sense to reveal him as a heel when they can hold off on it. Furthermore, it makes no sense to turn him into a villain if the company doesn't have someone in place to fill in those shoes.
Unfortunately for WWE, they seem to be switching between Rusev, Bobby Lashley, and Kevin Owens, which isn't going to be an easy decision to make. In fact, between Rusev having the fans firmly behind him now and Owens battling Rollins, it's hard to say who should be RAW's next big face.
Of course, a case can be made for both Superstars and even Lashley to some extent, but they need to find someone that can replace the hole left by Rollins after he turns into a heel.