Bethesda got lazy. They'll probably say that all vampire tribes followed "The Order" and made the deal with Clavicus Vile, but people will know they just didn't put the effort in.
It's hard for me to blame them though. Anything is better than what they used to have in the old days.
Vampire's only coming out at night and being shunned by everyone in the world pretty much breaks the game.... Have you played a vampire in vanilla Morrowind? lol
If they want other vampires to walk in the day light for gameplay and balance reasons I'm fine with it. They just need to explain it with a good story is all I'm saying.