The OP suggests that U+0243 B with stroke is a "capitalized alternate symbol for the voiced bilabial fricative in Americanist phonetic notation"
I would suspect that the other stroke capitals that are present were used in specific actual alphabets (and probably represented in some existing pre-unicode font or code block), whether 'Americanist phonetic alphabet', or other -- and the stroke capitals that are not present, were not.
In general, unicode codepoints are justified by actually existing use.
(There is an interesting metaphysical problem that arises as our lives become increasingly digitized -- things that are not present in unicode _won't_ be used, because they won't be _able_ to be used online... so how will new things be added justified by use, once all the existing pre-digital glyphs have been added? I dunno.)