Great question. The major difference is that you need to add people to Circles to see from and share to them, the entire UX is focused on Circles.

Often people don't use the list feature on Facebook because a core product assumption of Facebook is that everyone you add is your friend, and lists are not a priority in the Facebook experience. The Facebook sharing experience is also centred around engagement, where more numbers makes you look better. So users have the option of using lists on Facebook but there's little incentive placed there.

Lists also don't have much impact what you see. This means on Facebook the vast, vast majority of content still comes from people you don't really care about and random content accounts from businesses and influencers. So secondarily, Circles feed is simply chronological and only contains direct content from people you have added to a circle.