Hypothesis that porn producers in the earliest of days (pre-internet) found they could sell some niche tickle focused vids once they discovered tickle fetish was a sizeable enough group. They were already doing nude/mostly nude and then added some tickle content to bring in a bit more buyers.
Then the opposite in the latter half of the "golden age" of tickle fetish production. In order to draw in more than only tickle fetish audience, a fringe or non-ticklephile might still buy some tickling clips to see a bound hot naked model squirming and pleading for mercy without more painful or corporal torments. Moreso when it's F-F.
I imagine most native ticklephiles accept nude IF the tickling is genuine and amazing, but don't "prefer" it to clothed. The majority of folks over 40 saw their first "tickle FETISH" media with clothed ticklees. That's one thing but the other main thing is 99.9% of our real life platonic tickling experiences (even witnessing) don't involve anything close to nudity (swimwear most likely). If it happens with nudity for people IRL it is with a romantic partner and during or foreplay in intimate activity that would require nudity anyway. And surely there are romantic couples out there who are ticklephiles who go wild for spontaneous clothed tickle attacks mostly but schedule sessions where the Lee or both are nude as it exposes those additional private ticklish places (but may also involve direct sexual tease/play/genital tickling).
I'd also go out on the limb to say that most people here would say their most enjoyable/memorable platonic tickling experience ever was a clothed one.
Again, just hypothesis but there's mutual exclusivity for most fetishes. In a vacuum, I would feel safe saying overwhelming majority of ticklephiles greatly prefer non-nude tickle fetish media AND real life experiences. For exposure, lingerie, underwear, or swimwear/bikini but desire to keep the fetish play in that vacuum and not have it be sexualized.
That recent published (yet extremely limited) study of tickle fetish that came out last year drew fairly significant conclusions that ticklephiles in the sample considered the fetish experience of tickle play to give the euphoric pleasure and excitement of a sexual experience but did not need or desire their tickle play to be sexual.
YET of course, there's room for everyone's preferences and all have valid reasons. Plenty find nudity in tickling super hot and I'd never tell them it's not. But I'm listing why I believe nudity is not majority preferred by most ticklephiles. And this is more about the "fetish media" aspect than anything.
I've seen scenes both clothed and nude I thought were wonderful. But the ones involving nudity I enjoyed also involved other things than just tickling (D/s, sexual tease and denial, forced orgasms, etc.). Nudity solely with tickling (no matter how good) adds nothing for me. For ME, a grade A tickle fetish video of a nude ticklee would actually be MORE enjoyable if the ticklee were clothed. And that's because it ensures the eye and mind get to enjoy seeing the fetish in that vacuum of the fetish and not peripheral pornography.
Having a mutually exclusive foot fetish, it is the same for me. I enjoy seeing pretty feet on clothed women more in media and a nude model posing feet doesn't add anything and can sometimes be less enjoyable via the overall context of 'pornography'.
That said, one concept I have seen that I loved was the female ticklee was bound facedown topless with the premise that she had to endure being tickled while having to refrain from rolling to the sides (to avoid having her bare breasts exposed to the camera). It was consensual and that "predicament" added to the joy of watching without it being nude.