I can think of three possible reasons for the later start. One, they want after hours guests to actually be able to walk on or wait very little for rides at the event start, this added 30 minutes will help clear out the day guests. Two, it gives ticketholders the opportunity to catch Fantasmic or Rivers of Light before the event start. I wish they’d do this at the Magic Kingdom event too honestly. Three, being able to book the dessert party for the nighttime shows (which happen to start at 7pm) means more money in Disney’s pocket. Maybe it’s a combination of all of these reasons too, not just greed. Either way it gives you options and a better bang for your buck.