UK4Paul: It's a caucus, not a primary. Caucuses require greater organization, discipline, and an attention span that lasts longer than 2 seconds.
In a primary, you sign your name, go into the booth, and vote. Whereas in a caucus, you sign in, you're directed to a room, you sit (or stand) and watch people make the cases for their candidate, then you go off and vote. Not as many people would show up for that due to the short attention span of our country. A primary, however, accomodates short attention spans. Plus, taking time to caucus shows how supportive you are of a candidate, IMO.