For several years, the most popular configuration of a home refrigerator was to have the freezer compartment (also called the ice box) on the top of the unit as a separate compartment. As we know, hot air rises and cold air drops, so it doesn't really make much sense from an engineering point of view to put the coldest compartment at the top since it will require more energy to keep the compartment colder. The main reason this was done was simply for convenience. It was more convenient to have the smaller compartment with ice cubes in it at face level than down by the floor where a person would have to crouch down to their knees every time they needed some ice.
In the current age people are a bit more energy conscious. Some current refrigerator models do have the freezer compartment at the bottom of the unit in order to reduce energy usage and costs. But the most popular current model is the side by side with the freezer and refrigerator both vertical doors next to each other. This tends to make better use of interior freezer space. The ice maker is still at the top of the freezer compartment, though. This again is mainly a convenience factor. Ice cube dispensers in the door require gravity to help dispense the ice, so the ice maker must be high in the freezer for this to work.