Mainly because it changes the scope of your event if you advertise. Some insurers (SCCA's insurance, for instance) actually require you to specify if you're having a "spectator event" and charge you significantly more money.

But, even without that, if you advertise you're going to get a crowd of not just entrants, but spectators. Not something you want at a "regular" autocross, but it can work if you plan for it. It does take a lot of planning to do it right. As an annual sort of thing, it CAN be useful to draw new people to your events.