Why the export domain type is NFS only?

I see that it is possible to attach only NFS export domain to storage pool. But it takes far too long to copy VM or template data to such storage.

There is the corresponding note in a documentation:

"Support for export storage domains backed by storage on anything other than NFS is being deprecated. Existing export storage domains imported from Red Hat Enterprise Virtualization 2.2 environments remain supported. New export storage domains must be created on NFS storage."

And I am insteresting, that is the reason of such restriction?