Ukraine may not have more Neptun missiles. They were accepted into military use just weeks before the war started. Only 1 launching unit has been produced, and only a few missiles could be assembled from parts because the manufacturing facility was destroyed by Russia's cruise missiles in the first days of the war.
The scarcity may explain why Ukraine waited for 50 days to hit the cruiser. It was an "all or nothing" moment.
Also, there were reports that Ukrainian navy used 1 Neptun missile to hit a smaller ship, frigate "Essen", with less success.