Basically, because vampire myths were created long before science got that far. Vampires represent death, decay and disease, they feed off of life, but never directly, they feed on other people's blood, or breath, sometimes with additional requirements, only the breath of pregnant women, or similar.
On the opposite end is the sun, with its lifegiving properties. It is the ultimate representation of life itself, and because of that symbol of life, it's used to counteract vampires. Crosses work the same way, with Christianity getting involved. Can't have people worship the sun instead of our own religion, now can we?
Since then we've gotten very far with science, and some attempts have been made to justify the vampire myths with science, or to bring science into them, but the thing is, just like you can't come up with a scientific explanation for how Jesus returned from the dead and ran to heaven, myths are usually inherently incompatible with science, and that's basically the point.
If a vampire could just put on sunscreen, if they were just hurt by UV-radiation, then they're not very scary anymore. They're not supposed to make sense, they're not supposed to be explained. The interesting part of a vampire is that it IS mysterious, that it IS supernatural, and that it DOES defy science. It's not the radiation we can measure that hurts the vampire, it's the symbolic representation of the sun that damages it. It's not the biological elements of blood that makes vampires feed on humans, it's the lifegiving properties, and our lifeblood that sustains a vampire. You can't use a uv-light to hurt a vampire, and you can't grow a substitute source of nutrition in a lab, that goes against the very point of the myth.
That said, I welcome new additions to the mythology, I love new interpretations, but in trying to explain with science, you ruin what makes the myth interesting. There's zombies for that.
