Why do climbing shoes have 2 loops?

To help pull the heel of your shoe over your heel from two angles, this helps keep the shoe open when you put your foot in it and get the shoe over your heel.

Think about a garbage bag, it's easier to put something large in it when you open the bag from multiple angels rather than just holding it from one.

The same applies to climbing shoes. You are stuffing your heel in a very tight shoe that is much more difficult to put on compared to regular shoes. The same logic will need to be applied here as with the garbage bag, you will need to open the shoe up from multiple points, rather than just pulling one part of it, which would just make it actually more narrow and harder to put on.