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For the same reason that they also include symbols for many obsolete (dead) languages and writing systems, as well as (per a comment above) a character used by the 1959 IBM 1401 computer (https://github.com/shirriff/groupmark). The need to be able to discuss a certain technology in writing does not disappear just because it is out of general use.

They have symbols for languages that _have_been_ obsolete for centuries, if not millennia.

Also, if popular, these kinds of things tend to outlive their technology. For prime examples, look at http://graphemica.com/, http://graphemica.com/%F0%9F%96%AA, http://graphemica.com/%F0%9F%96%AB, and http://graphemica.com/%F0%9F%96%AC (to be fair, I think that is overdoing it)

I found the proposal with the new symbols here: http://www.unicode.org/L2/L2015/15238-n4671.pdf

They were proposed by Japan, and intended for TV electronic program guide. If it makes you happier, one of the symbols is for 8K.