One because AC is more prevalent in first world countries, and people are not as outside oriented as they used to be, both in labor and recreation.
Two we have not had sweltering 100+ degree summers like in the past for much of the country, such as the '30's. We here in the Florida of Iowa have only had about 10 days in the 90's this year, and nothing above 95.
Better clothes, better awareness, easier access to portable water, ice, coolers...the list goes on.