If we were to take a stab at reading the filthy subconscious thoughts of the Russia-haters on this one, we'd say it's a combination of actual, demented hatred of Russia (the Estonian expert mentioned above, for example, is likely predominantly driven by this), geopolitical requirements to demonize Russia (that one's a no-brainer), and the postmodernist agenda to destroy 'good clean fun' precisely because therein lie values which are antithetical to liberalism - which, in the West these days, is synonymous with nihilism.
It would be interesting to see, if Masha was transgender and the Bear an animal rights activist pushing veganism, whether Western elites still had a problem with the cartoon...