I think it's pretty clear why they created Fuchsia:

1. They want a modular microkernel OS with modern capabilities.

2. They want to eventually replace their internal Linux based OS with Fuchsia.

3. Fuchsia, or parts of it, will be the basis for all of their products.

4. They want to control the direction of their own OS.

A lot of people speculate that that's why Flutter was written in Dart instead of Kotlin or something else. Google wanted to use a language they already have a lot of investment in, and for some reason didn't pick Go. Which honestly seems odd to me since Go can be compiled all the way down to binaries already, they had to invent that compiler for dart, but whatever. Dart is super cool and I'm looking forward to using it in Flutter's user space.

Dart supports generics, has a good package manager and their devs are open to the history of programming language's design and modern tooling.

Go isn't interpreted, making hot-reload impossible.