Baking or Cooking - why orange and not other sweet citrus fruit?
I don't claim that this is a canonical answer, but it seems to be a bit like the situation with apples. You have cooking apples and eating apples, which have been bred for different traits. Similarly you have juicing oranges, eating oranges, and bitter oranges (used for marmalade). The other citrus fruits which taste most like oranges (mandarins, clementines, tangarines) are more suited for division into segments, and you're more likely to see them used for decoration. You could juice them if you don't have a better option, but recipes will tend to use the most suitable commonly available option.

Grapefruit has a very different flavour, so you have to take that into account when substituting. It's also not necessarily sweet, and certainly I have never seen grapefruit sold as "sweet grapefruit" or "bitter grapefruit".