The difference is that Stalin is an officially desgnated demon figure and the Russian nobility are not. Ergo, in the reflexively Stalinophobic world, a portrait of him is automatically BAD, whereas portraits of Russian nobility may be fine art. Stalin = Bad.
WS:
My RiP grandma who was born before the 1917 revolution shook in horror on the very thought of the Russian nobility. Autre temps autre moeurs.
Wojtek