Comment by dzaima

Because NaNs come from a standardized hardware-supported type, whereas the rest of those are largely language-specific (and you could consider null/nil as a "not-a-*" type for those in applicable languages; and there are languages which disallow NaN floats too, which completes all combinations).

Itanium had a bit for "not a thing" for integers (and perhaps some older hardware from around the time floats started being a thing had similar things), so the idea of hardware support for not-a-* isn't exclusive to floats, but evidently this hasn't caught on; generally it's messy because it needs a bit pattern to yoink, but many types already use all possible ones (whereas floats already needed to chop out some for infinities).