The allure of xmlhttprequest was that over connections much slower than today, and with much less powerful desktop computers, a user didn't have to wait for the whole page to redownload and re-render (one can argue that focusing on better HTTP caching on the server and client might have been smarter) after every single user interaction. This was also much of the draw of using frames (which were also attractive for some front-end design use-cases later re-solved with CSS).
As apps got more complex, clients got more compute, bandwidth grew, and as web audiences grew, offloadingl much of the page rendering to the client helped to both contain server-side costs and increase or maintain responsiveness to user interactions.
Now, as desktop client performance improvement slows down (this isn't just the slowing of computer speeds, computers are also replaced less frequently), average bandwidth continues to grow, app complexity and sophistication continues to grow, but as server compute cost falls faster than audience size grows, shifting rendering to HTML back to the server and sending more verbose pre-rendered HTML fragments over the wire can make sense as a way of giving users a better experience.