Because migrating to NextJS would have meant doing a rewrite of the entire backend which was already written in Python/Django and running in production. Migrating to htmx was much less risky.

I don't think the point here was to show how quickly they could migrate to another framework, but rather to show that HTMX is powerful enough to use for things like this.

The point seems to have been to show how you don't need to use React to build applications like this, so migrating to NextJS wouldn't have made any sense from that perspective.

Perhaps they are better equipped to write more Python, less JavaScript.