It worked for many years, so why change to advance ticket only.???
1. Banked non-refundable revenue, easier to estimate total revenue of event and book acts accordingly
2. Liquidity ahead of the event, reduces dependence on loans
3. And the big one: Simplifies ( and hence reduces cost of ) liability insurance by guaranteeing that no more people can enter the grounds than the number for which tickets have been sold and preventing surge attendance.
Relatively recent trend for aishows but common in other branches of 'entertainment' show. Which really illustrates the direction in which irshows have been headed over the past decade.