For the same reason they signed Woods and traded for Watkins: he has questionable hands, runs sloppy routes, probably still doesn't know the entire scheme at X/Y/Z, and it's easy to miss him with that miniscule catch radius.
If you watch any offense run by the guys who taught Sean, you'll see precise routes and steady hands as a basic requirement. So IMO at least you don't really need to ask the staff, the truth is he doesn't fit and they don't trust him in that role over better actual wide receivers.