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.