#3 Having Roman Reigns win would have made fans upset
WWE already made a questionable choice by having Charlotte Flair win the women's Royal Rumble instead of creating a brand new star. Had Roman Reigns won the men's Royal Rumble, there would have been a riot and an uprising from fans, and they certainly wouldn't have liked it at all, honestly.
And the best part is that on SmackDown, Roman Reigns can still go on to face The Fiend, now that his feud with King Corbin seems to be over and done. There is enough time and more until WrestleMania to build the feud and The Fiend will certainly be on for a lot more shows than The Beast Incarnate is, I'm guessing.
I mean there's always the possibility that Drew McIntyre will go on to face The Fiend at WrestleMania. However, I think that's a very unlikely scenario and the match with Brock Lesnar seems extremely likely right now.
Also, as amazing as Edge is, I don't think he's ready to be slotted back into the title picture just about yet.