Because it's a drop-in, no-dependency, no build-step library that can make your static MPA a little less static for the little bits of interactivity you need with not much effort. It's what you reach for to avoid "write the whole website in JS."

React -> "The site exists in JS, HTML is just a render target."

jQuery -> "Poke at the HTML from JS." Which is brittle as hell.

htmx -> "The site exists in HTML, extend HTML to handle the common tasks you want to do with it."