I found a talk with some history of the Spotify client's transition:

https://www.youtube.com/watch?v=9UsnX5X_DF0

It was originally native, then native with some web-based bits, and then the web-based bits slowly ate the whole thing. It seems the ease of development with web-tech compared to C++ (also the better font rendering and screen reader support) was just too tempting.

Interestingly the Spotify client was never based on electron but instead arrived independently at the "chromium + web app" architecture at around the same time as Atom did.