I don't know what Google's purpose for Fuchsia is, but Linux was originally designed for a security model which is uncommon these days. The Linux security model protects different users from each other, but these days it's much more common that a computer will only have one user, and you want to protect that one user from potentially harmful code.

Capability-based security is a big step in that direction. I don't know what "they're both open-source" has to do with it, obviously there are other reasons to choose between different pieces of software besides the license.