PC has spoken admirably of Koch Industries and he thinks there's some merit to remaining private. That's probably why. I don't have the source at hand but probably one of his podcasts.

I feel like it makes sense for Koch to stay private since their work mostly focuses on fossil fuels and specifically fracking. Public markets can be a big distraction with protests and what not. Is stripe worried about being cancelled?

Your guess is as good as mine but they likely thought that the status quo in market would continue.