Why is Sing not available on 2nd generation Apple TV 4K?

Because it isn't compatible to "Sing" on Apple Music With Apple TV 4K (2nd Generation) It has to be in Apple TV 4K (3rd Generation) Included.

As this is a user-to-user community, no one here can tell you why a particular feature isn’t available.

One difference is the A12 chip vs. the A15 chip, which brings a lot more processing power to the later model. That is my take on it.

The question was why is it not compatible with 2nd gen. Why does it have to be 3rd gen? I wish there was an unhelpful button