Because High school programs can be hotbeds of Politics & Drama and it is really not worth putting up with for some girls.
In some cases you are taking all the drama that happens in the halls of the average High School and putting it on the bench of their favorite sport and why should they put up with it when they have a travel team they are happy with.
To answer your last question Collage coaches don't really care a wit about high school -
And sometimes the HS coach IS the drama...