- We make a ton of mistakes. Undoing those mistakes was harder than it needed to be.
- Rapid development and testing of features was massively more important than browser support
- PHP was just talking via HTTP to a RESTish API anyway, and behaved like a shifty middleman.
- Search engines were going to struggle with indexing us
- Once we let go of server-side processing outside the API, development was much faster. This is important when users give feedback that you can factor into features.
- Cheap to serve. Theoretically, the site could be downloaded and ran locally. The major costs are all in providing the API.