Turnout at an association event requires an active invitation strategy. Members choose for themselves whether they come. They're not staff members who are required to attend.
Three things make the difference. First: a clear value proposition in the invitation. Why is this event worthwhile? Second: a low sign-up barrier. Make signing up easy and free (or cheap). Third: multiple reminders. Send it at least six weeks in advance and add two reminders.
Add social proof: 'Already 400 members signed up.' Involve regional chairs in the communication. A call from your own region works better than a central newsletter.