Based on what's been posted on the boards, Royal is using all dining rooms for early dining. So all of the tables are "full". When people start clearing out from Early, they can open tables up for MTD. But, that's generally not going to be until 6:45p or so. At least, they can't count on having tables available before that, so they can't take reservations.
Why they can't start seating 30(?- maybe 15) minutes after Early starts based on what tables are available/who didn't show, I don't know.