That's a very good question, John, and one without a better answer, I suspect, than the limits of practicality. So many topics for philosophical reflection, so little time! As a matter of practicality, many philosophers feel the pressure of researching and publishing in the more traditional philosophical categories, in the interest of a respectable and marketable curriculum vitae.
But like other "philosophies of" areas of ordinary human life, like food and wine, philosophy of sport seems to be gathering a number of citations in recent years. The Philosopher's Index returns 189 hits for abstracts published since 2001 with "sport" in the title (a better indicator of topic than if "sport" appears anywhere in the text), and there is a semi-annual Journal of the Philosophy of Sport, which also began publication in 2001.