what causes this to happen

Maybe your carb has a low leak and the fuel is draining itself out. Here is AUS it gets quite hot and I can have my car sitting for a long time without pumping the buggery out of it to make it start. I also have no choke connected - threw that away, waste of space. Also check that your primer squirters are functioning properly, if your plunger is worn or you have a blockage - you'll have the problems that you are describing as well