I can't speak as to their motivations specifically, but one of the things that I've noticed from poking around the kernel API is that the kernel APIs are much more coherently designed than the conventional kernels we have are.

In particular, one of the key design goals appears to be a heavy use of a capabilities-enabled handle-based API, even for more conventional syscalls (e.g., mmap). One of the benefits of this approach is that it simplifies a lot more cross-process management stuff; you can inspect (or edit!) another process's memory maps, for example, with the same system call that a process would use to edit its own. It would also enable something like CreateRemoteThread; it would definitely be far easier to write a debugger for Fuchsia than it is for Linux.