*sighs* I don't know why.
My best guess? The first series of Type 29 nekos, the NH-29A, had it. Maybe Wes judged the skin holography little used and decided on a NH-29B series lacking it.
Mind you, according to Wes, the NH-12Bs stopped existing too, despite the Sylph II power armor (a very recent model) being based on being piloted by a NH-12B.