Why reCAPTCHA V3 is not available?
Version 3, is completely invisible to the user and will never ask the user to complete a task, on the form submission it will only assign a score from 0 to 1 to the request (where 0 means most likely a bot and 1 means most likely a real user), base on the user or software should take an action.
For example, a version 3 practical application could be used to assign the score to a submitted comment, and the WP could either put it into moderation or approve it automatically.
On the other hand, if reCAPTCHA version 2 is unsure if the person submitting a form is a real user it will make the user complete the challenge to authenticate the request.
This means version 2 will work better with things like registration and Ad submissions which is where the WPAdverts reCAPTCHA integration is being used.
Additionally please note that at this point Google is not planning to retire version 2, it will be maintained/developed on par with version 3.