It's not. In some dialects H would be silent, and you can certainly write "an historic" because of that in informal writing, but it's discouraged in style guides that sites like this would use.
This is incorrect information. It’s actually quite controversial and most learned people point to “an.” Either way, one shouldn’t talk factually as if the issue is settled. Particularly if you’re citing style guides, like you are in this response, rather than informal usage.
It's a dialect issue. Learned people in different regions disagree.
Me, I voice my 'h's, so "a historic" is correct in my locale.
When style guides recommend you to use "a historic" in formal writing, why should I not talk factually about what they recommend. You were the first to complain about the usage in this article.
“An ‘istoric”
Ironically, that's how it'd be transliterated in greek - where h really is a strange exception that gets a breath mark rather than its own proper letter in the alphabet.
It could also be because the H is almost silent in some English accents.