Because it’s the Black Sea, much like the US’ Great Lakes. It’s only got one entry/exit point and it belongs to someone else. There’s nothing else for them to do unless an enemy force enters the Bosphorus Straits, which is owned by Turkey.
Underway at sea, less so.
The Black Sea Fleet was launching quite a few missiles at Ukraine, was a considerable blockade threat, and also a significant alternative logistical supply to Crimea. (Escorts for freighters and / or use the landing ships for logistical supplies.) Once Ukraine gets ATACMS, even a modest number, it’s bye-bye Kerch Bridge, and ditto for the rail routes through SE Ukraine to Crimea.
I’ve seen it suggested Ukraine might choose to not take full control of Crimea, for now, due to manpower considerations, but, it will be functionally lost to Russia, and whatever you may think, I’ve been listening to the Russian leadership about Crimea for at least 20 years: That’s a BIG deal to Russia.