Without knowing more about the application, I'd guess probably caching and/or scaling. If you only need 1 payload then that can be statically generated and cached in your CDN. Which in turn reduces your dependence on the web servers so few nodes are required and/or you can scale your site more easily to demand. Also compute time is more expensive than CDN costs so there might well be some cost savings there too.
This was basically it. The dataset was the same across users so caching was simple and efficient, and the front-end had no difficulty handling this much data (and paging client-side was snappier than requesting anew each time)