Simply because it was an adverb, even if it was from a different language and has a perfectly clear noun meaning.

“ad nauseam”, we were originally using a different term for this (and it was semi-commonly used) but after finding this phrase in an unrelated context I felt it fit best, as it was much less questionable and also a lot more general

“tedium ad nauseam” was proposed as a compromise, but I was very much unhappy with there being a problem in general

Didn’t really matter since was already declining sharply in use and I don’t think the concept has been referred to within this year at all