How do I see their allow list

Probably it’s a whitelist (likely to prevent trolls and such from using fly-by-night instances to spam them). That list can be viewed at https://hilariouschaos.com/instances - then click on “Allowed Instances”

or if they’ve not allowed piefed where do I see why not?

Unless they posted publicly in a community somewhere, you may not be able to see that - you’d have to ask them.

That said, it’s likely the former - just they didn’t notice piefed.social until now because it was so small compared to some of the other instances on that list.