So, why is it that vampires in Skyrim and Cyrodil can walk around in broad day light, but the Vampire Clans in Morrowind die from sun light?

I read Immortal Blood and tried to make sense of it. But none of it really talked about the "why" on Sun Light. I do remember something being mentioned about Clavicus Vile mutating vampirism where they could walk around in the day so long they were well fed. But nothing explains why the Volkihar can too.

Is is possible that Clavicus Vile made it to where all vampires can walk around in the day light ?

Not all vampires are the same. Different clans have different abilities, including whether or not they burn in sunlight. I don't know if it's ever specified why sunlight affects any of them, how it all works etc., just that not all clans are affected by it.

Yeah, I'm very interested with how they came up with it, as sun in TES universe is actually a hole through which magicka flows, which should mean that vampires should be extra weak to spells or something, and yet they aren't, and in fact, most of them use spells like crazy.

But yeah, there are many vampire clans, and a lot of them have different traits. Skyrim vampires are supposed to be able to "walk" through ice, for example.