In my experience it's boiled down to the type of data you're working with. Building nested, tree-like structures and then submitting that structure to the back-end as one request is more easily done via the front-end than a bunch of back-and-forth requests followed up by a "commit" request.

edit: I suppose this is different concern than a true SPA, but as another sibling comment points out, its just a matter of time before routing makes its way into the front-end as well.