I'd guess primarily the lower volume of the PoE HAT would play a role here. But, it's also a fully isolated power supply with a fan. To me, it looks like this

I'd guess primarily the lower volume of the PoE HAT would play a role here. But, it's also a fully isolated power supply with a fan. To me, it looks like this couldn't be made on a fully automated line, so there would need to be manual assembly, which is also more expensive.