Why do I need to enable ranking in order to create a new scum sprint?

[Stan Etra]

Seems like ordering by anything other than ranking means that iterative development is not possible...

[Nic Brough [Adaptavist]]

Because the rank is what determines what you need to be pulling into the next sprint.  It encourages iterative development because you can draw in what you need to do based on all the data,not just one aspect of it.  You might think ranking by age or priority are useful.  They can be, but only when you start out with a new backlog.  They're not a lot of use once you start to work iteratively, and they're certainly no good on their own.  If you sort by age or priority, you simply can't account for dependencies or other work.  If, for example ABC-234 is urgent, but ABC-123 is trivial, the priority is fine.  Until you realise you can't do ABC-234 until ABC-123 is complete.  The way around broken sort orders like that - rank them.