Comment by Maskawanian

It certainly has an optimized kernel for its use case. I believe it also includes ZFS by default. I wouldn't be surprised if the Proxmox developers would prefer to upstream these defaults, but they likely would introduce regressions for the common use case that Debian optimizes for.

Ultimately, I use Proxmox as a hardware hypervisor only, so I don't mind that it uses its own kernel. Everything I run is in its own VM, with its own kernel that is setup the way I want.