Or did they choose not to as that would have antagonised Russia?
Probably: Ukraine & Ukraine's army was in a different state in 2014 and everyone thought Russia had the world's third leading military. Right now, the Russian army is demoralised, untrained or dead with half its usable tanks gone. It could use nuclear weapons but the western response would definitive.
From what I have heard (caveats; not an expert, happy to be wrong), Crimea is served by water from Ukraine (which is why the Kherson dam is a big deal), a trainline (which I don't think Ukraine controls...yet) and a pair of natural land bridges, plus the Russian route.