It's not just about borders. Ukraine has some Russian Empire's/USSR's most fertile land. It was facing serious overpopulation (i.e. peasants with no land to work on) in the end of the 19th century, while Russia had a lot of unpopulated land in Siberia and the Far East, facing imperial Japan and potentially China.

Ukrainian peasants were offered free land and transport if they moved east in an attempt to solve these two problems, and many used the opportunity.