Roman Reigns has been the top merchandise seller in WWE of any “full-timer,” if John Cena is counted as a “part-timer” (Cena is still ahead of Reigns otherwise), according to the Wrestling Observer Newsletter.
The fact that he sells so much merchandise is likely a major reason why WWE hasn’t turned him heel yet, with the idea being that he is very popular as a face with kids, and turning him heel would likely significantly bring down his merchandise sales (at least in the short term).
However, the feeling among a lot of fans has been that a heel turn would be the best thing for Reigns, since he would likely eventually emerge as a much less hated babyface after a successful stint as a heel.