This is probably why Spotify doesn’t implement it. It must be purely business reasons. The technical aspect is easy.Spotify on HomePod is the biggest missing feature. I’m not willing to buy an Apple Music subscription for just one device so I AirPlay Spotify to HomePod. Unfortunately, there is a lengthy delay.
The reasoning one can only speculate . I’m guessing they don’t want to create more room for hardware manufacturers to control consumers than necessary. Just like why Google Stadia failed.