Basically at the core of it they wanted to optimize the development loop:
1. Make a change.
2. Compile.
3. Test.
4. GOTO 1.
I'm personally of the view that they wanted to optimize the development loop for someone working predominately from the command line in essentially vim, without a language server. I know a bunch of devs like that and this reminds me of them in a lot of ways.
3/
Early versions of Go outside of Google had no package manager to speak of and it _still_ has a relatively weird stdlib. It made assumptions about your development environment that largely were true at Google, but nowhere else.
The early gen garbage collector also wasn't really all that and had weird pauses that were difficult to debug. There was a nice enough concurrency model, but it wasn't earth shattering and it didn't solve any of the actual hard problems to speak of.
4/