I just saw a long play of this on youtube. Looks like a fun shooter and I wouldn't mind having a copy in my collection, but I saw in the video comments that the game goes for over 600.00. A quick ebay search doesn't show any except for a repo cart that was $99.99.
Why is it rare? Low production count? One of the last games out of the gate for the system? A hundred bucks for a repo cart seems outrageous, but I would be open to owning the repo for a reasonable price.
I don't know much on what is rare on the Megadrive.