It has to do with physics and heat. To try to explain it would take a large post and even then, some people may go, huh? I flew the B-767 and was able to fly up to about 124 degrees. Smaller planes are more affected by high temperatures than the larger aircraft. I occasionally would fly into Sky Harbor in Phoenix during the summer and never had the issue that some have now. In 34 years of flying, I was only grounded twice and both times were in Dallas.
I know that people who had planned vacations become very upset, but normally, they do get out of town by the end of the day as the evening begins to cool the temperature as the sun fades. The larger planes, like the B-747 are rarely affected by temps.