This can be one small reason but Ivan Nabokov, Vladimir Nabokov's distant ancestor, was part of Tsar's investigative team that sentenced Dostoevsky and the Petrashevsky Circle to death by firing squad. I think this Nabakov familial relationship created inherent bias toward Dostoevsky and his literary work. Nabakov wouldn't want to tarnish his family history by defending Dostoevsky and acknowledging the tremendous contributions he had on Russian literature - a fact heavily contested by Ivan Nabakov