Why?
1. All vampires of the provinces are noticeably horrendous. No matter how much they feed, they will always appear monstrous and obviously be detected as vampires. The exception was the vampires of Cyrodiil who can blend in mortal society undetected if they were well-fed. Not to mention they prefer to feed on sleeping people and the vampire gameplay in Skyrim is similar to Oblivion, you feed on a sleeping person. Let me make it clear that Cyrodiilic vampire's ability to blend in is unique to their clan...JUST THEIR CLAN.
"To patron http://www.uesp.net/wiki/Lore:Clavicus_Vile, beacon oer our affairs, we owe our successes and social stature. Our bond with Vile makes us unique among our kind, for his guidance steels our savage craving with reason and savvy. For him we live amidst mankind, and twist them to our will from offices of power."
"Their true name was unknown, lost in history, but they were experts at concealment. If they kept themselves well-fed, they were indistinguishable from living persons. They were cultured, more civilized than the vampires of the provinces, preferring to feed on victims while they were asleep, unaware."
2. Stages of vampirism: The longer you go without feeding, the stronger you become and the more obvious your vampiric nature is. The powers are similar as well, Embrace of Shadows and Vampire's Seduction are powers returned from Oblivion. There are few new vampire powers, however.
3. In the book titled "Immortal Blood", we are told of a vampire hunter named Movarth Piquine who enters a chapel to inquire about the various types of vampires. The man in the chapel gives him advice, and so Movarth learns and goes on his hunting tasks in different provinces. When he asked about the vampires of Cyrodiil, the man in the chapel told him they were indistinguishable from living persons, more civilized than the vampires of the provinces and that they prefered to feed on sleeping people. Movarth returned to the chapel after a month, frustrated that his search proved fruitless. The man in the chapel eventually reveled his true nature, but for Movarth it was too late, the vampire had already attacked him. Where am I going with this? Well, he was attacked by a Cyrodiilic vampire right?
Spoiler
Movarth Piquine is an NPC in Skyrim in a quest called "Laid to Rest." he is known to be a vampire master who has other lower vampires masquerading as citizens in the town of Morthal. It's no coinscedence that the book Immortal Blood is found in that same quest in the home of a vampire working under him. So it leads me to believe they indeed are in Skyrim. But the is no lore-based books to support it as Manifesto Cyrodiil Vampyrum did on Oblivion.
Also, it appears Porphyric Hemophilia was renamed to Sanguinare Vampiris. So did the Cyrodiilic vampires evolve in these two hundred years? Or is there a new explanation? For the people that do care. What gives? Why are there Volkihar NPC's then?
