“The feature that makes space special is locality. Interactions between different objects happen when they are nearby in space. Two billiard balls bounce off each other when they come together at the same spatial position. Nothing of the sort happens when particles have the same (or opposite) momenta; if they’re not in the same location, they just keep going their merry way. That’s not a necessary feature of the laws of physics—we could imagine other possible worlds where it wasn’t the case—but it’s one that seems to hold pretty well in our world.”