Before 1990, Ukraine had existed as an independent nation only a few times, most recently for a few years after WW I and for a brief period in the 1600's. The country had been under total or partial Russian control for most of its history and as a result, a large number of Ukrainians felt a kinship with Russia as they also spoke Russian.
Couldn't the same thing be said about Poland, Slovakia, Czech Republic, Hungary, Lithuania, Latvia, Estonia, Finland, etc.? They were under the control of either Russia or the AHE for most of modern history. All Slavic languages are similar. If you're fluent in one, you can get by in the others. A people doesn't need defined borders. If they did, there would never be a Jewish Israel. They didn't have a homeland for multiple centuries.