Why do climbing shoes have 2 loops?

The two loops are meant to help you pull your shoe over your heel. Sometimes it can be quite difficult to pull the shoe over your heel, especially for those who choose to downsize their climbing shoes, and having just one loop may just make it difficult to pull it over your heel, and could also shorten the lifespan of the loop since it alone will have all of the pulling tension. Eventually, it can tear.

Adding another loop allows the tension to be more spread out, rather than all of the tension being in one point. It also allows more area of the shoe's heel to be pulled, which helps make pulling the shoe over your heel easier.

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.