Why?
1. All vampires of the provinces are noticeably horrendous. No matter how much they feed, they will always appear monstrous and obviously vampirism. 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.
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?
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.