The center or league chooses the format of the league, not USBC. You can have all men, all women or any combination. There are leagues that require two men and two women, others just one of the opposite sex and of course the all men or women leagues. The one of the opposite can be one man or one woman. It is again up to the center or the league. In our center,I believe the owners decided what they wanted to promote and that has changed over time.
As far as why women can bowl in mens, but men can't bowl in womens. That goes back to the ABC and WIBC. The American Bowling Congress had to allow women, since they are Americans. The WIBC, Womens International Bowling Congress, did not since they were a Womens only organization. Had the ABC forefathers ever thought that day would come, it might have been the American Men's Bowling Association or something like that. I would venture that since the merger, the seperation is strictly a choice that the USBC has made. If the Women's nationals were made an open event, there would be two open nationals. That wouldn't make any sense, so they keep one open and the womens event. Many local associations are still not merged, so they have the two events as well. Independent tournaments can do whatever they want. My events are open, but once upon a time we had a Men's division and a Women's division. I ended the women's and created the open format due to lack of entries in the women's only portion but many tournaments still offer both or an open and womens.
