Jack Vettriano – the self-taught Scottish painter – is a hugely popular artist in the UK. But no public museums have interest in showing his work. Why? “The proposition that Vettriano cannot be slotted into a spectrum of art that runs from Titian to Damien Hirst and Sarah Lucas is just risible. Why Pop Art but not Popular Art? If the National Theatre can reinvent itself by staging popular musicals and also populist shows like Jerry Springer – the Opera, then why cannot our galleries do likewise?”