It's because the fans and the club indirectly through their incompetence expect him to be a super hero all the time. I've mentioned here that over the years we usually went into big games with Rashford as basically our main hope of getting something. The recruitment has been so bad that i don't think we signed a single winger on his level or better while he was here, so we'd usually go into every season expecting Rashford to hard carry the team again. You obviously mentioned what happened when he failed to deliver, endless insults and criticism when in some of those games he was the only one who looked liked he had the level to play in Man Utd's forward line. He's just an easy target.
Nevermind that the team is shite, tactics are crap and he plays with teammates who can barely complete a dribble or pass a ball properly, if he isn't at his absolute peak he gets all the stick anyway. I also find the lazy Rashford narrative a bit dishonest. Yes there's something to be said about his defensiveness contributions in recent years, but how are we labeling a player lazy when he was one of most willing runners?
There were games were through just watching him I'd estimate he made about 15 high intensity sprints behind the opposition backline, sometimes carrying the ball from deep because we didn't have a single midfielder who could carry or pass properly bar Bruno. In fact he used to be our main counter attacking threat. Well he isn't here anymore and he'll do everything not to come back. Goodluck to him.