The main reason that baby back ribs are so expensive is that they are in high demand. Everyone wants a little bit of the rib rack, and there are just fewer baby back ribs to go around.
Baby back ribs are tender and lean. They are also smaller than spare ribs. A rack of baby back ribs tends to contain between 11 and 13 ribs, about enough to satiate one person. In comparison, a rack of spare ribs can satisfy 2 people.