I had a similar experience shooting amateur (i.e. club level) motor sport. Sales were very disappointing. However, I have a theory.
The thing is, I used to do this before, back in the late 80s. That first tim around I made quite a bit of money, albeit using a different sales method (pre internet & digital). I would first of all go round the paddock asking who was interested in receiving proofs, then I would make contact sheets - yes, this was back in the Dark(room) Ages! I would mail these proofs to each driver and wait.......
This time around I've had much less success, even though I reckon my photos are better than my earlier efforts. There is always interest in the paddock (I still go round collecting names and car details so that I can send a 'heads up' email when the images are on my website) and there is always a flurry of site traffic in the week after the emails go out, but sales are low.
My theory is that people are much more used to being photographed these days, if only by family and friends, and let's face it, even compact cameras in the hands of amateurs can produce quite reasonable pics these days. Then there are the enthusiasts who own fairly decent DSLRs and are happy to give their work away, simply for the appreciation they receive.
Added to this, the plethora of web sites covering every single event around the globe means that most people get to see pics of themselves published from time to time. The result of all this is that most people really aren't that keen to actually PAY for photos any more (unless of course you're lucky to grab a real 'money shot', such as them and their car flying airborne over the fence or some equally heroic image).
The most common exceptions to this apparent apathy are competitors who need pics for their sponsors, or mothers of junior drivers. Even then, there simply aren't enough of those exceptions to make the business pay.