Board Thread:Lore Discussion/@comment-2165692-20160701070121/@comment-28235247-20160807141144

I really don't think that there's anything to the sun weakness. Bethesda knew that they would be no vampire if they weren't weak to sunlight, and maybe they needed a nerf.

Or as Heroic Mist said, something could happen to the magicka coming out of the sun. It doesn't explain the fire weakness though, all undead are weak to fire if I remember correctly.