Because Clavicus Vile demaned it.By this information on Tes:wikia we may be able to explain the vampire problems:
From the "Manifesto Cyrodiil Vampyrum," we can take a note that Clavicus Vile altered the Vampire blood of Cyrodiilic Vampires, made them be able to walk during daylight and most importantly able to deal among the common folks as another commoner - if they were well fed. This was actually a great gift for the Vampires, and quite possibly a turn against somebody wishing to fight them. Who Clavicus Vile tricked with this great service towards vampires is unknown.
What i don't understand is how vampire's in skyrim also walk in the Daylight...or I'am wrong?
Edit: This happend around The Third Era:433.I don't if that was the time of Obvilion,but could also explains it...

In other words kids complained about dieing when outside at daytime so Bethesda changed it.

FUUUUUUUUUUUUUUUUU.But that was on morrowind right?

your wrong, these are not the same vampires from cyrodill (The Order, the only ones affected by that thing with the prince), no evidence that supports that idea (The Order would be really easy to find if you know what to loo for, mostly vampires in positions of power, which there is none i think)

It makes no damn sense. Vampires of Cyrodiil are the only vampires able to blend in. The ice junkies known as the Volkihar are shown using the same thing, unless only NPc vampire are Volkihar and PC is Order.

Poor design choice.

They could of at least backed up lore: Cyrodiilic vampires migrated to Skyrim after the Empire weakened. Maybe they took over and wiped out the Volkihar by manipulating villagers and the Imperial Legion patrol and have come to set their attention on the Thalmor.

That was easy enough. Manifesto Cyrodiil Vampyrum wasn't even that long, a simple book explaining that could of been enough to back up gameplay.

Lazy is the choice of words here.

Doubt it. The Volkihar Vampires are still in, and a Volkihar Vampire can kick the crap out of a Master Vampire. Pit a Volkihar Master against a Master Vampire, and the Master Vampire practically gets 1 shotted by the Volkihar Master. Volkihar could do the same to any Order Vampire.
The "Sanguinare Vampiris" strain is likely the result of yet another daedric pact, with the Daedric Prince, Sanguine. So it's not exactly the "Order" in Skyrim, but similar. The Volkihar allow them to survive because the Volkihar don't care about the cities/towns, which is where this "Sanguinare Vampiris" strain seems to hang out near.

kids are too impatient to play morrowind

I love it when somebody trots out this stuff

I played Morrowind. I remember playing Star Wars when it hit the arcades. And Dragon's lair. And Berserk. And Hunt the Wumpus. And Pong.

Do I win the internet now?

Yeah...that's why the Cyrodiilic vampires ousted all other vampire competitiors before. And there is no proof the pact was made with Sanguine. The word Sanguinare has to do with blood, rather than a Daedric Prince. Also, it causes wonder why a couple of vampires went to ask Clavicus Vile for a cure rather than Molag Bal..or in this case, Sanguine. Yet in the Clavicus Vile quest, they worship him just as the Order vampires do.

Gameplay wise they may be more powerful, but lore-wise, Cyrodiilic vampires have the advantage.