VMware and Virtualbox both rely on kernel-specific modules being loaded in order to work their magic.
But twice in the past 7 years it has been so badly broken that it can't be fixed with a patch, leaving users with a choice to either run their VMs on an older Linux platform or find some other virtualization package. I'm glad I went through some pain to switch to KVM -- it seems quite solid.
Hmmm...was that mostly an issue on Debian Testing or Unstable?