Part of your low-volume plan sales might be the perceived complexity however to me, looking at your chassis picture, it seems fairly straight forward. Certainly the steering has a degree of complexity not found in other, more traditional karts.
I have noticed that a lot of the kart-plan sellers on ebay aren't having their auctions close out to a winning bidder, and I think part of that is the economy - adults, who are the ones with the money, are not spending as much on non-essential things in preparation for the "worst".
I also think with all the free plans that are available, there are enough alternatives that someone might not take the risk of time and materials trying to make your kart. Not because it's complicated, but because it *looks* complicated.
I'm in my 40's and a parent, and I cannot tell you how many toys and things my four year old "absolutely had to have" that now sit in a pile in the attic. I can't imagine what this will be like when he's 12-15.
My personal theories aside, I have to admit your kart design looks great and is quite striking. I always appreciate unique approaches to anything. Kudos.