WordPress as a CMS is fantastic, lots of end-users know and love it. However, the dynamic nature of any website you build with it is not ideal, particularly since a large number of WordPress sites do not need to be rendered for every request.
The benefits of static websites are key for most use cases WordPress users have. At the moment, they have to spend a lot of time fine-tuning performance and SEO. Gatsby comes with all of that out of the box, and is cheaper to host and scales infinitely.
Specific to Gatsby, it is easier (and will only get easier) to hire JavaScript engineers than to hire PHP engineers nowadays. The development experience is also much better.
Further, Gatsby will be the best “no code” tool.