There’s really only one reason, but perhaps it bears repeating: it didn’t work as intended because it was scripted (and badly). Her delivery is flat, her laugh lines aren’t self-deprecating and it sounds like the sort of partisan in-joke that only the really committed would find funny (and only then because it’s repeating something they all think is “true”).
The thing that worked about the so-called crying incident (and the aftermath) is that, regardless of whether it was or not, it seemed real. It seemed unscripted. There’s not a woman in the world — particularly those of us who are sorta hard-ass bitches — who hasn’t had someone be just sort of nice to you at a moment of frustration or exhaustion or vulnerability and felt the water-works start to go. That’s what’s humanizing about it, and that’s part of the root of the frustration about the hyperbolic reaction to it. But a scripted attempt at humor isn’t humanizing, it’s just sort of annoying. I’ll bet Hillary probably does have a sense of humor — about herself even — but like many women (me included) it’s more contextual and less about written jokes. The thing about being “humanized” is that you have to, you know, act like yourself and you have to sort of not give a shit if people like that. Or at least just tell a fart joke. Those are always funny.