I've given this a little thought over the years.
And I have yet to come up with any definitive answers. But I think at least in part, it's because it's such a
STRANGE thing to be aroused by. Somebody finds out it's your kink, and you're afraid that they're gonna be all,
"Tickling? Really?!? TICKLING?!? You can't be serious! Embarrassment, maybe? I mean, come on... this IS weird, no matter which side of the equation you're on. 'Lees-- They enjoy being touched in a way that makes your body involuntarily go through all sorts of convulsions and spasms, they crave a touch that, once it begins, they're
DESPARATE to get away from/stop from happening. Let's face it... that's just strange desire. 'Lers--we're just as odd, wanting to be the one causing all those things to happen. I can't even describe my desire to do this to
MYSELF adequately, much less to someone who
doesn't get it at all! So yeah, I think a part of it, at least, is being a little embarrassed/ashamed of our little kink being such a simple but strange thing. In the mainstream media, if someone mentions whips, chains, spanking, BDSM, etc... with regards to sex, they don't really even bat an eye anymore these days. You pair up sex and tickling, and they're like, "What?" It doesn't occur to most.
Their loss.
EDIT-- What RUNOFFSTAGE said made me think of something else, too. If people who know you KNOW that you're into tickling in the bedroom, that totally throws any innocent tickles that happen, the regular horseplay, out of context, and they might start thinking that you're getting your jollies from something that's really innocent.