As someone who implemented a SPA framework prior to "SPA" being a word much less React or Angular, I have to say for my company, it was all about state management.
Distinguishing between web apps (true applications in the browser), and web pages (NYT, SEO, generally static content), state management was very hellish at the time (~2009).
Before that, pages were entirely server rendered, and JavaScript was so terrible thanks to IE and a single error message (null is null or not an object) that it was deemed insanity to use it for anything more than form validation.
However, with the advent of V8, it became apparent as an ASP.NET developer that a bad language executing at JIT speeds on the browser was "good enough" to not send state back and forth through a very complex mesh of cookies, querystring parameters, server-side sessions, and form submissions.
If state could be kept in one place, that more than justified shifting all the logic to the client for complex apps.