Whether something is up or down is more meaningful in terms of our average interaction with that object than whether it is left or right. Conceptually the latter is inherently relative to the observer, and in many circumstances the observer could just turn around to change the state of whether it’s left or right, so it doesn’t matter which way it is. Whereas they can’t do as much about many things that are “up” or “down” like the sun, a tree canopy, the earth’s mantle. Those things are more constant generally so it’s easier to grasp them