“All big letters but it isn’t no acronym” - DOOM, _Ballskin_

I think it's part of the aesthetic of being a comic book villain, because comic books are typically in all caps.

The traditional comic book style lettering is also ALL CAPS which may have influenced his preference https://i.imgur.com/Po3IJAa.jpg

Because he wrote it that way.

Because it's a scream, and all caps are a way to express screaming in text.

it's from his song All Caps.

"Just remember ALL CAPS when you spell the man name"

I think part of the all caps thing thing is world building and relates to the graffiti influence in hip hop. All caps being and hand style used in many graffiti tags (and the MF DOOM tag). With DOOM a lot of the time you have to infer the meaning of a rhyme from what you know about DOOMs influence and his other works.

"He write "VIK" in a sick demented handstyle" -Viktor Vaughn (another alias), Raedawn.

Or ‘Monkey Suite’

“DOOM, all capitals, no trick spellin'”

“Got what it take to get it through your thick melon”