Because Rein is not easy when played at maximum potential. His skill ceiling is pretty high.
Anyone can pick him up and use his mechanics just fine, but getting a hang on shield management, his ability combos, best times to use his ult, the best ways to protect people, etc. These are learned things outside of straight mechanics that raise his skill ceiling and make him show great value and can help identify the difference between a good and bad Rein.
I myself, am a bad Rein. My shield management is not good, I suck with his abilities, etc. I can stand there and hold up a shield, but I won’t compare to a good Rein.
This is what old Mercy offered in the same way. Low skill floor, entry level. But a higher skill ceiling provided through thought and ability management.
Having weaknesses such as team reliant mobility, minimal ways to protect yourself or contribute via damage as much as others, a single target healing or damage beam that you have to make the most of to make up for that loss in damage, the thoughts that came with making sure you used Mass Rez without resulting in your team and yourself, dying.