Why depends on the person of course, but some like it because it’s a deep wiring association from development (often seeming like an illogical manifestation), some like it because they’re extremists of some kind (like how some people like the heightened thrill of roller coasters, haunted houses, or sky diving), and some people like it because they look for edginess or novelty to keep the spark alive for them. Humiliation and traumatic themes are not just an aspect of the tickling kink—they tend to be on their own axis of interest and overlap with a lot of different kinks. In fact, humiliation and degradation by themselves are some of the most popular kinks right now!
I personally like it from both the lee and the ler angle. I’m not really into degradation, but humiliation can appeal to me at some level. It definitely isn’t my thing as much as it is for many people, but there’s something about the risk and the power the potential embarrassment has over me that makes it equally as exciting. Tickling is one of the very few things that has ever been embarrassing to me, so maybe that has created a deeper intrigue over time. Playing with the idea of vulnerability is a big thing for me, and both humiliation and more intense fantasies really just spark that feeling within me.
For the “traumatic” stuff, I get into some of it and some of it I don’t. It can be very case-by-case. A lot of it can absolutely arouse me in fantasy, but the thought of it being in a reality context immediately turns me off. I imagine this is similar to how those with extreme kinks like rape fantasies are, which has always been a little difficult for me to fully grasp, but through awareness of my own separations of fantasy and reality I’ve started to understand it more.
Another thing that might have something to do with it for some of us is that as children (in a reltively healthy upbringing without abuse, etc), one of the worse things we might have imagined is being tickled to an extreme. I think that gets into a lot of our heads and carries forward into adult fantasies that take it even further. I imagine there are as many specific reasons as there are people that are into it. But it’s a very common fantasy element all across the kink world.