Russia put troops into Crimea before the referendum, and the referendum was run by the occupying army. Do you normally trust occupying armies to run referendums about whether or not they should get to keep the land they’re occupying?

97% in favour of Crimea joining Russia. Western polling was a solid 70%+. The new 2014 regime was legitimately divisive to the point that the majority ethnic Russian populations in Ukraine did not want to submit to them.