It turns out that licensing doesn't work that way.
Worldwide distribution costs more than domestic, and streaming rights cost more than broadcast-only. And PBS doesn't have as much leverage as for-profit broadcasters, so the negotiations for those rights took longer.
Here's some prior context:
https://www.usatoday.com/story/tech/talkingtech/2018/04/15/w...
https://www.techhive.com/article/3338083/why-live-pbs-stream...