The general rule for prioritizing the backlog is that you do it based on business value. From the business perspective, this makes a lot of sense: you maximize the ROI by doing first the things that create the most value in the context of your product, be it monetary or non-monetary (e.g. number of visitors on the website, number of registered users etc). You define the goals during the release planning and then you make sure that you’re going in the right direction.
The second level of prioritizing happens during the sprint planning meeting. The team looks at each story and tries to see if the story is ready-ready, meaning if the Product Owner says it’s ready for the team and if the team thinks it’s ready to start. So actually the highest business value is not useful at this point anymore; instead, there’s something else.
It’s about information.