In art, portraits have had a poor time of it since the Second World War. Many came to think of portraiture as at best a minor form of art, inferior to the grander modernist traditions - such as minimalism, conceptual or action art - that ask the really important questions.
When you look at the art usually done under this label, the assessment seems accurate. After all, portraits are often largely painted to flatter the sitter, or at best to make observations about modern life and social attitudes. The latter kind would include artists like David Hockney or the (to my mind awful) portraits of the Glasgow Brutalist school.