Missing episodes Apple TV+

Seems like that specific episode was removed. You would need to contact PBS and ask them why. Its entirely up to the rights holder’s when or how they make their content available.

You can try to find it directly through PBS rather than on the TV app.

https://www.pbs.org/video/episode-2-m4o7rg/

This is a user-to-user forum; no one here can answer any question that begins with “why”. And it’s also an iPhone forum, which has nothing to do with Apple TV+.