I’m not entirely sure, but perhaps it is because “incarnate” doesn’t so much *describe / qualify* the evil as simply state what form it is in. To me, even though it’s a single-word adjective, it feels like a relative clause (“evil that has been embodied”) or a prepositional phrase (“evil in human form”). The word “incarnate” (literally "put into flesh" i.e. "embodied") is borrowed from Church Latin, and is mainly confined to Christian religious contexts (in your example, it is used hyperbolically). Note that in Latin, the adjective usually follows the noun. Another example of noun before adjective is the “Attorney General”. Here, “general” does not describe the “attorney”, but merely lets us know of his government office. This expression was borrowed, like many legal and financial terms, from Norman French.

In the context of religion, especially Christianity, ‘incarnate’ can come both before and after the noun. It’s a rare word outside of theology. There are other words with very similar meanings. He is evil incarnate. He is evil personified. He is evil in the flesh. He is evil come to life. To me, Phil’s expression seems to be on the right track. It’s not a mere adjective describing a type of evil but a word describing a process where an abstract concept transforms itself into something different. So it merits different treatment.

It's a foreign word (latin) and in latin languages the adjective comes after. It's an artistic word that you won't see often unless the writer is trying to be very dramatic. An "embodiment of evil" is the same meaning as "evil incarnate" but embodiment is an english word, not a latin word so you would see it before the noun. Unfortunately because of invasions by the romans and french, sometimes english has a few words which mean the same thing (cease/desist).