Back in 2004 there were a lot of tickling sites but not nearly as many as there are today. Since there weren't as many sites, GIFs, images, etc. it may have taken longer to find the specific niche of the fetish that we were looking for. Therefore there was a lot more searching for the term. But today there are more sites with more content as far as videos, pictures and whatnot. Not only can people find the content that they like quicker but many of us already know what sites we like therefore we don't need to randomly search the word "tickle" as much as we used to.
Not sure if this is the reason but it's just my guess.