By contrast, Haiti appears to be of zero importance to the US, so my guess is natural resources/geographic position.
I always assumed it was about Crimea specifically and naval dominance in the Black Sea. Also the natural resources embedded under that sea, along with Ukraine's other resources which are not too shabby either. Although now much of the chernozem ("black soil") has been mined and is littered with decaying military equipment and dead soldiers, so this one might be rendered useless for a good long while.