Because some features of KVM (eg. KSM) does not work with OpenVZ kernels. This is discussed already a lot of times, e.g.: http://forum.proxmox.com/threads/1991-Survey-Proxmox-VE-Kernel-with-or-without-OpenVZ
We have now four different kernel branches, see http://pve.proxmox.com/wiki/Proxmox_VE_Kernel
So is it up to users' choice to use any of those 4? Will you be supporting all of them?