Why

Same thooooo

Maybe it's because Alastor and Lucifer look very alike?

How?

It's simple: they're brothers plus they have the exact same smile

Hmm

Same smile, same fancy dress style, same smirks, plus Lucifer is a good version of Alastor and Alastor is an evil version of Lucifer.

Isn’t Lucifer the devil?

In myth lore, yes THE Lucifer is the devil, but he's a fallen angel in Hazbin Hotel

Huh

I have a hunch that he was originally from heaven but sent to hell for some reason, because as you can see white is the color of pureness and good nature, and he of course wears white, and Alastor could possibly be his brother canonically due to some similarities.

Understandable

Fallen angel, actually.

Lore-wise incorrect.

You can almost call me-
DAAAAD