On any threaded fork, the threaded portion has to extend far enough so that you can adjust the top race. Often forks with longer than necessary steer tubes are not threaded far enough for a shorter head tube. As already noted, the threaded portion is not as strong as the threaded portion of the steer tube. This means that even with a longer steer tube and spacers you cannot raise the stem and higher than you could with the steer tube cut to the minimum length