Low turnout at corporate events almost always has the same cause. The promise was not strong enough or the logistical barrier was too high. Both are solvable.
Strengthen the promise in the invitation: not what you present, but what the participant takes away. Lower the barrier with three communication moments. Send an invitation four weeks in advance and a reminder two weeks before the event. Three days before, send a practical message with directions, parking tips and a short agenda. People who are uncertain about the logistics drop out sooner. Also make sure sign-up is simple: a complex registration system costs you participants before they even get there.