A rectangle shape gives a more “even” bounce across the whole of the jump mat so no matter where you jump on a rectangular trampoline, you can expect your bounce to have equal height and power. This happens because not all of the springs on a rectangular trampoline are used when you jump. Instead, the springs work independently of each other depending where on the trampoline you are bouncing.
Have you noticed that all competition level gymnastics take place on rectangular trampolines? A rectangular trampoline has an even bounce across the whole surface of the trampoline, giving more control for serious gymnasts and athletes.