"I may have to take you up on that, then. The other ingredients I have plenty of; vampire dust, not so much." It's almost as if the vampires decided that the man who took down Harkon in single combat is probably not someone they want to mess with, thus denying him chances to collect any. "And I don't doubt we'll be in need of healing potions and spells for at least a little while yet, considering the sicknesses that seem to be going around."

And there's also the little fact that the outbreak seems to have coincided with the arrival of a man who can make others sick by his very presence. Could be a coincidence, but he rather doubts it.