I think we have covered this before. Here’s my take:
Using kernels provided by dom0 makes it easier to control and update
that part of the Qubes “infrastructure”.
(If it were necessary to patch the kernel in some way, then a single
update would be enough, rather than having to generate patched kernels
for Debian, Fedora, Ubuntu, Arch, Gentoo etc.)
It’s also far easier to troubleshoot problems if one knows that affected
qubes all share the same kernel.
And it’s consistent with the general Qubes approach, of providing a
reasonably simple backend to the process of creating and using qubes.
When I comment in the Forum or in the mailing lists I speak for myself.