Anotov An-225 Mriya: Reports largest plane destroyed at Hostomel airport (heraldscotland.com)

Probably because just being one of a kind doesn't make it valuable during a war when they have to evacuate more important assets

Priorities. If your country was being attacked you'd prioritize handling military equipment. This plane, while being absolutely extraordinary, is not an important asset in a war.

There was not a lot of notice that an attack would happen.

No time or resources can be allocated to be sentimental about the plane, maybe. It's a war. Not the most expensive thing that's being destroyed right now

Gee, I do not know? How about them perhaps prioritising the survival of the population and overall being somewhat “busy” in a state of national emergency? It seems perfectly logical to me that this would take precedence over the protection of a plane largely useful at peace time, no matter how unique it may be.

Seems to be confirmed now:

https://twitter.com/DmytroKuleba/status/1497947370008547332