It’s because of where the turning wheels are on the car. It’s much easier to get the turning wheels to go sideways in a short space, than it is to get the fixed wheels to go towards the curb. By backing into the spot, you get the fixed wheels as close to the curb as they need to be, and then you can relatively easily get the front wheels near the curb because the front wheels can turn towards the curb without turning the whole car towards the curb,