Answer:
Why is the word Holocaust considered a proper noun?
Holocaust: If the spelling is with a capital letter, it is a proper noun.
This is the most common way in which the word is used.
it refers to the systematic extermination of the Jews by the Nazis during world war 2.
bonus:holocaust: With a lower case spelling can reflect to any similar situation and would be a common noun in this context.
why is it always capitalized ?
the word Holocaust is capitalized, to show that it is a proper noun and also shows that it is one of the most racial misunderstandings in the world.