... games became a matter of routine during World War II, almost certainly as a matter of national patriotism.

It had been played at World Series games and other special events on an occasional basis, even before it became the National Anthem in 1931.

For more information on this, see:

http://tampabay.devilrays.mlb.com/NASApp/mlb/tb/history/tb_history_nations_anthem.jsp

Other sports most likely followed along with baseball after it became apparent that people liked it (as they surely did during the '40's and '50's).

----- jim o\-S