In alternatives, packages write their files to the disk and then register those as alternatives. A user can decide which one should satisfy the alternative.

In diversions, a package claims a file. When anyone else tries to overwrite the same file, it's diverted elsewhere. It isn't designed for the user to choose between equals.

I built a derivative distro using diversions mainly to prevent "soft-forking" Debian packages as much as possible (maintaining packages is extremely costly for anyone that isn't a decentralized organization like Debian)