No one has mentioned it: mobile! Mobile apps became a thing, and there was a strong argument to have one common backend for your web app and your iOS app and your Android app. Plus the mobile UX (especially with iOS apps at the time) was such a gamechanger that there was a natural shift to replicate it in the browser.

That said, even though I still build SPAs at work, but I can't wait for the day that I get to build something big with Django & htmx.