Perhaps because in order to get Ukraine to give up the then third largest stockpile of nuclear weapons in the world the US in the Budapest Memorandum on Security Assurances in 1994 gave Ukraine security assurances against threats to its territorial integrity and political independence?
I assume I don't need to explain why the US wanted that stockpile to go away.
One might argue that now that Ukraine does not have that huge stockpile the US should just leave them to their fate. But the US still wants to convince various countries to get rid of their nukes or curtail their attempts to develop nukes, and letting Ukraine fall would not instill confidence that the US would hold up its end in any deal to make that happen.