My guess is it’s because PyQt. AFAIK there’s not a JS binding (or any other language really) for Qt that’s nearly as established and steady. If you’re going with Qt, Python is the most reasonable choice (other than native C++ of course).
Historically it was a much more productive choice, than just about anything. Today it's merely an arguably more productive one. In any case it remains a good choice, and one of the most popular languages.
Despite years of FUD from the likes of Microsoft and others, "investing in a second language" is not a big deal at all and just short of required for projects of any complexity.
What libraries should I rely on to create cross-platform native (or native-approximating) UIs in JS?
Javascript is quite commonly established as a good UI language these days, and I assume that anyone who can write Py can write Js.
Is there something particularly useful about Py, i.e. perhaps researchers who know Py may not want to bother to switch? Or that it integrates better with some things? Or that it's used widely at a company and investing in a second language is a big deal?