Problem is that historically Russian territories (with predominantly Russian population) was added to current arificial Ukrainian state.
Somehow it never bothers Russia when historically non-Russian states with predominantly non-Russian populations are added to the artificial Russian state....
With regard to ethnicity and language the situation is a lot more complex (leaving aside the issue of how to decide a person's ethnicity, self-identification? Soviet nationality assignments?)
And which territories... in the Donbas, Russian predominated in the cities but the countryside overwhelmingly spoke Ukrainian until very late USSR (this from someone with relatives still there who had no problem with being in Ukraine).
The only people I've personally known from Crimea both claimed Ukrainian as a native language when there was political pressure to do so (pressure not to do so for one of them). Another person who knew people there said those who had adapted more or less to capitalism had no problem with being in Ukraine it was the welfare dependent classes who dreamed of bigger government handouts who wanted a Russian government...
I understand that Odessa is traditionally almost entirely Russian speaking but more Ukrainian identifying....
de-facto prohibition of Russian language even on territories inhabited almost 100% by Russians
Some citations might be in order... it's pretty normal in this part of the world for a country to want to develop their own language.
I kind of agree with the American (forget his name) who supports Ukrainization but suggests that Ukraine also standardize it's own form of Russian, making their own dictionaries, formal standards etc. drawing a parallel with Spain and Mexico. At one time the standards prevailing in Mexico came from Madrid while now, Mexicans don't care what Spanish think of the way they speak.