Ordinary people believe whatever they are told by purported authority figures. If Russians are told by the Kremlin that Ukraine "really" belongs to them, 99.9% of people will automatically believe it because that's the path that requires the least intellectual effort. Moreover, anything that leverages nostalgia is almost certain to appeal to the great mass of ordinary ignorant foolish people. Just look at what nostalgia for the 1950s did in securing Trump's first presidential term in the USA and what nostalgia for the Empire did for Brexit. Unfortunately, there's no way to educate ordinary people, which is why so much of human history is a tale of endless stupidity and self-harm.