It's not that. It's a combination of the guidelines discouraging insubstantive comments -- to keep the signal-to-noise ratio high -- and genuinely good jokes being hard to do.

Most of the time, if I try to crack a joke, it gets downvoted because it's insubstantive and not all that funny. Once in a while, I've gotten a fair number of upvotes for a humorous remark, such as this one under my old handle:

https://news.ycombinator.com/item?id=14995810#14996229

IIRC, it hit high 50s for total points.