That's two reasons! (1) It's offtopic, which is especially bad given that the threads are so comment-heavy; and (2) it's too easy to exploit.
I suppose exploit is not the right word, because a lot of it is people posting their grudges or bad feelings that they probably have perfectly legit reasons for, and yet are not the full story. We're not going to get the full story, or even a fair assessment, from a distracting back-and-forth in the middle of a job ads thread; and that's ignoring the point that most people posting to Who Is Hiring aren't in a position to respond to such complaints in the first place.
It seems to me easy to see that if we allowed it, there would be no limit to it, and the end result would be a bad scene indeed.