I've always found the popularity of Granny Smith in particular to be bizarre. They are really horrible apples, good only for cooking. Cox's Orange Pippin is superior in every way including a much more interesting and rounded flavour.

Actually, I do have a theory why it's so popular. It's because of the phrase "an apple a day keeps the doctor away". People regard them as medicine rather than a treat, so it makes sense for them to be disgusting.