I think you misunderstand what isolation does. It will also keep the heat out. Having window awnings to block sunlight from directly warming up the room will go a long way.
The problem is if you can't get the home cooled off, then heat that does build up can't escape.
In western Canada until recently we got away with very good insulation coupled with opening windows at night as daytime-vs-nighttime temperatures could swing as much as 15C.
But as both daytime and nighttime temperatures increase, or in more humid climates where the temperature doesn't drop enough, that's no longer effective.
The net result is the home just gets hot and stays hot.