The market for a generic would probably be smaller. My guess is each application probably shares basics (like an accelerometer) but has different requirements (accuracy, noise, processing power, sample rate) if not entirely different sensors (maybe one needs GPS and one needs a barometer). Which means the generic bracelet that would work for every application would have to be the superset of all possible requirements- and that's $$$$.

But you probably could market one bracelet for a couple different sports, so long as they are similar & you can write the software. Baseball & tennis might overlap a lot, for example.