Why do vampires never die?

Vampires never die, because they are already dead. Vampires are considered undead, which means that they are deceased, but behave like they are alive.

But "undead" Vampires can be killed but lots of things. Most of which has changed over the years since the original Count Dracula story.

They (apparently) can be killed by :

Pure running water

Fire

Putting a stake through the heart

Decapitation

Sunlight

Denying them access to their burial soil (Usually in their coffin).

receiving a mortal blow using a silver implement

The reflection of a crucifix (or any other holy symbol)

Garlic is a major irritant but in high dose can cause death

More modern films and books have mentioned a Vampire disease which can be used to kill them.