As many other comments are saying, because it's Go.
The majority of developers who use static-site generators professionally are frontend developers. Their language of choice is javascript or typescript. And if many are complaining about the inconvenience of writing graphql plugins — in javascript! — for Gatsby, imagine the annoyance of having to write the logic for your build step in an entirely different language. In Go, or in Ruby for that matter. It isn't fun. You can't easily use the skills you developed in javascript to tweak the behavior of the static site generator to your liking. You can't extend its behavior by writing a custom plugin. You feel constrained, and at the mercy of Hugo maintainers. It's very disheartening.
However. There is a fantastic alternative both to Hugo (by being javascript-based) and to Gatsby/Next (by not requiring React for the simplest hello world). Its name is Eleventy, and it is beautiful!
Hugo is a single executable - it doesn't require any knowledge of Go itself. Granted, you have to be familiar with the templating syntax to add build-time logic to the website, but I believe that syntax is common across many other static site generators as well.
Yes, if you want to add logic in addition to the templating primitives provided by default, then being able to write code into the source pages will be of great help. However I think that when such a level of dynamism is required, then one can start debating the utility of a static generator in the first place.
The majority of developers who use static-site generators professionally are frontend developers. Their language of choice is javascript or typescript. And if many are complaining about the inconvenience of writing graphql plugins — in javascript! — for Gatsby, imagine the annoyance of having to write the logic for your build step in an entirely different language. In Go, or in Ruby for that matter. It isn't fun. You can't easily use the skills you developed in javascript to tweak the behavior of the static site generator to your liking. You can't extend its behavior by writing a custom plugin. You feel constrained, and at the mercy of Hugo maintainers. It's very disheartening.
However. There is a fantastic alternative both to Hugo (by being javascript-based) and to Gatsby/Next (by not requiring React for the simplest hello world). Its name is Eleventy, and it is beautiful!