Pixel hardware is some of the easiest to get/best/cost effective with an unlocked bootloader.

They only support Pixel hardware because it's currently the only line that meets their list of requirements. I'd guess that if something came around which beat the Pixel line, they'd support it...but I also don't see anyone positioned to do that right now.

Though it's worth mentioning that the developers don't emphasize degoogling all that much and their community often have a bit of tension with degooglers who come to join them. The OS certainly meets the needs of those of us getting away from Google but the developers have no problem recommending workflows that go through Google (albeit with regular app access rather than the privileged and deeply integrated access on stock Android) when they're more secure than the alternatives. For example, they'll regularly suggest using sandboxed Google Play over F-Droid or Aurora Store, again because of their stance of prioritizing security above all.

It can sometimes be a bit annoying when your priorities are more about avoiding corporate surveillance than protecting yourself from attackers or a snooping government, but their work ends up supporting both regardless.

I think because on other manufacturer android phones, like Samsung, you'd have to de-Google and also de-Samsung.

I'm betting it's because Pixels have some unique hardware/ software quirks or something.