Because if they rush it into stable, and then find flaws in it, or want to change how an API works for whatever reason, it will break applications that used to work perfectly one compositor update ago. (not so stable behavior)
When you first transition to Wayland, you need to replace applications that don't use Wayland APIs and it's a pain in the ass. Now imagine having to do that every time your compositor is updated. That's what rushing things into stable will do.