I suspect it has to do with the way sails are cut. In order to get a properly shaped triangle the luff has to be shorter for a jib.

With a larger genoa you also might want to take advantage of the better air up high.

You could use a taller jib, as long as its not too tall, accounting for stretch.

This is for a hanked on jib. For a furling genoa, the luff dimension is critical, although the sail can be hoisted higher with a pennant at the bottom