A few years ago an old (RE experienced) HS coach once told me that "sometimes the best players are sitting in the stands" At the time I did not understand what he was talking about.
Now that I am watching my son and what is happening at his HS, there are many reasons for kids not to play a particular sport. Playing multiple sports takes a lot of discipline that some don't have, others want to focus on one sport, grades, family issues, the list is long. Some HS coaches are out recruiting athletes from other sports (within the school) however our baseball coach is pretty straightforward saying he only wants the kids that want to be there.