I would have to say the USA tirals are the most difficult tournament to win, more than nationals, or even the world championships (especially indoors) Unfortunately it is difficult for everyone to attend. Dont forget, most of the top collegiate shooters from the past couple years are now out in the real world and cant take off work to travel to a competition.
However I can not blame the low attendance by top shooters at nationals to the USAT. It is a NATIONAL championship, I would think USAT would not be a major factor.