In this part of the world, the DC's had historically been scheduled during July & August, months when children are normally away from school on summer vacation.
I understand that over the past decade or so, many summer conventions have been scheduled earlier, often in early June when most children are still in school. While parents of younger children may opt to remove their children from school on Friday to attend the DC, teens may not have that luxury as they may need to write final exams at that time.
In this manner, the WT is responsible for the decline in attendees, particularly on Friday.
As far as conditions are concerned, JWs today have far less to endure with a 3 day, 10-5 program, often held in an air conditioned venue.
When I was growing-up, summer conventions were 5, 6 and sometimes 8 days long and would end each night at 9pm except for the final day. As venues were usually baseball or football stadiums or horse race tracks, attendees would remain outdoors in full sun & rain while sitting on hard, uncomfortable wooden slatted seats or benches while dressed in suits and dresses.
As there was usually only one convention designated for a large area, there were few options to split the convention over multiple weekends. Even if another convention was located relatively close-by, attending it would have been considered a rebelious act as anyone doing so would not be attending their assisgned convention.