In this case, I don't think it's necessarily a big issue and it certainly isn't the abandonment of a platform. There is now an enormous range of third-party products with built-in Chromecast support. It's entirely plausible that Google looked at the current market for Chromecast devices and concluded that the Chromecast Audio was no longer necessary. It was already a somewhat niche device, but it has only become more niche as third-party support has grown.

https://www.google.com/chromecast/built-in/audio/