It's a marketing issue. People likely to get hit with ransomware are incredibly unlikely to understand what that means. Hell, even main devs have trouble writing contracts, so even if a user knew there was a smart contract, verifying it would be another thing. So it'd get reduced to "guys on Twitter said this one works".

I like the idea though.

Since you can't store the private key needed to decrypt the files in ethereum, I can't think of how to do this.

All blockchain state is public, since it needs to be calculated by and verified by all nodes, so there's nowhere to stash a private key without revealing it.