Is a great artist one who develops and progresses?
Is that why he is not regarded as a great artist, even though millions of people are reputed to buy his paintings on greetings cards and as prints?
Maybe he has secretly progressed and we should see a large exhibition of his, to show us that he deserves the same attention in the art history books as Picasso*
(Maybe, but I don't think I'll go to it!)
*an artist greatly wronged by a critic who once claimed he can't draw. After seeing the fabulously small and perefectly-formed Picasso collection in Barcelona, I am absolutely stunned by his drawing skills.