What happened to all of Chicago's conventiongoers? McCormick Place has seen nearly a third of its business vanish this decade, a period that saw two recessions. Attendance at major events in Chicago's main convention center declined from 3 million in 2001 to 2.3 million last year. Las Vegas and Orlando held up somewhat better but likewise have seen attendance fall off since the middle of the decade. But there's a silver lining for Chicago in its loss of two large conventions this month: It shows that the competing cities have space to fill.