Rock can't wrestle a full match. He's not in ring shape and Roman's not in a position to carry him like that. Like
@Mantis Toboggan M.D. said, Rock hasn't wrestled an actual match in eleven years, and he wasn't in ring shape then, either. I know that this match has been on the table for years. I also know that Cody has to finish the story. You need a way to hide Rock's limitations and also give the fans the ending they want, which is Cody winning the title and taking down Roman once and for all. Doing a triple threat allows you to do both those things.
Storyline wise, if Cody is willing to step aside completely and let a 50-year-old man take his WrestleMania main event, why wouldn't he be willing to share it? Especially when Rock has no claim to a title match and he's only getting one because he's The Rock? And if Cody can just give up his WrestleMania opportunity, that devalues the Royal Rumble. There should at least be a rule in place that guarantees that spot for someone else if the winner decides to vacate it.
But you know what would have made this whole thing easier? Not having Cody win the fukking Rumble.