Because ORD is very extremely overcrowded in terminal 3. You will wait more tehan 10 minutes to get cleared security. You don't want stuck in the lines. You will missed the flights but, airlines will rebooked your next flight out from ORD. That's why there is no more room for security screening in ORD. I actually that we cleared security before I headed back home to FLL years ago. It was no lines there wait more than 10 minutes to get cleared security.