It's not really about Zelensky, he's basically a figurehead, like most leaders in capitalist countries. The capitalists (Ukrainian and foreign, mainly Amerikan) and the military are more or less in control of Ukraine at this point (Zelensky himself was heavily funded by one of Ukraine's largest capitalists). As for why they care about retaining Crimea, I'm sure there are economic reasons for the Ukrainian capitalists, Amerika is likely just using it to ensure that there won't be a peace deal, and there are certainly reasons for the Nazis who control the military; if you look at what they've done to people they viewed as Russian collaborators throughout the conflict and what they've been doing to the people in the Donbas, you can imagine what they'd do if they got a hold of Crimea