Unread and uncited are very different things. It appears that the 50% number is some sort of arbitrary (short?) time scale "has it been cited yet" metric. Citing something is much harder than reading; you need to do some research, write it up and go through the publishing process. If 200 people read a paper but not cite it, is it a failure?
I've often tried to find the "canonical" paper to cite for a certain fact, and found that it barely even mentions the relevant fact at all. It just somehow became canonical because everyone needed a citation for the same thing later on.
