All the time my man. Showtime and HBO do this quite a bit with just an hour time slot and one match, the main event. They do not show the undercard. PPV matches are entirely different and that is not what I am talking about. Most boxing matches are NOT ppv. Therefore, the hour time slot works best for the networks that carry them. It's all about money.
Okay - maybe we're seeing some different fights. Even fights that I've seen on HBO/Showtime where only one fight is shown, they still block out 1 and-a-half to two hours, and spend half the time talking about each fighter. Also, 15 rounds should only be for title fights, so all these matchups where titles are not on the line would be excluded.