Because this conflict has zero, to do with ethnicity. Rather, it is an attempt by Russia to come to the aid of those who identify with Russian culture, whatever their ethnicity. That includes a lot of Ukrainians.
One man's "invasion" is another man's liberation. I don't believe the folks in Donetsk see things in quite the same way as you are attempting to portray them.
The aim of the Western elites, since well before 2014, has been to turn Ukraine (and eventually Russia) into a Slavic version of Canada; a liberal, woke, fascist state where the proclamation of the Christian gospel, especially as it relates to human sexuality, is deemed to be "hate" speech. In short, Russian intervention is aimed at liberating those who have a problem with Western/American culture and don't wish to see rainbow flags hanging off every building. So when I say "Russian culture", I mean a culture where the ruling elites are not lead around on a leash by sodomites.
In short, this is a "culture war" which has turned into a real war. NATO is simply the military wing of Western cultural imperialism. To portray this situation as simply another land-grab by Russia is ridiculous but fits neatly and lazily into the American world-view, circa 1962.
Ukraine is a client state of the US State Department but Russia won't go quietly....at all.
That's Moscovite thinking, because they see themselves "owning" all the states that were ever part of their empire. They can't even imagine a state refusing "ownership" by Moscow without assuming that somebody else (i.e. the USA) must be "owning" it. And you're falling for it.
Carry on, Putin lovers.