Because Julia is even more awesome ;-) No seriously there are lots of great programming languages out there, but they all have different strengths and weaknesses. Go is very nice for systems programming, server side programming etc.
However for working on algorithms, data science and engineering and science problems it is quite bad. If I was writing a database, compiler, web server or crypto currency, then I’d probably chose Go.
However if I want to write a quick script, process or visualize data, write a simulator or solve a coding challenge like in project Euler, then I’d much rather chose Julia. I also strongly believe it is a great teaching language. It is both user friendly while allowing the user to be exposed to wide variety of concepts within progamming: functions, closures, types, functional programming, abstractions, macros etc.
If I was writing an operating system kernel then I’d chose neither of them and probably use C or Rust.