I get the impression that Vue 3 was a greater effort than expected. It was talked about for years before beta release (on Full Stack Radio, for example). March of 2020, I was at Vue Conf (days before Covid lockdowns began), and I heard a few people complaining that this wasn't the expected official Vue 3 release. Here we are almost two years after that and it's just becoming the default.

That said, I understand that there were monumental under-the-hood improvements, some of which spawned off Vite.