The leftist art establishment viewed Rockwell paintings as very successful propaganda posters for the system that they were and still are trying to destroy. He was highly technically proficient, as any professional commercial illustrator has got to be in order to make a living and remain employed. His color palette was filled with light and positively glowed. His composition was excellent. His communication of emotion and attention to detail was above reproach. So, they had to look elsewhere to hate him, but at base it was political. A fair amount of their esteem for any given artist is due to his or her backstory, and from that they determine intent. Rockwell was being paid to churn out beautiful, inspiring depictions of everyday American life and did so flawlessly, much to their ongoing dismay.
Nicely stated!