I think it's because people are used to listening to their Playlists. It feels more natural to check your "Discover Weekly" playlist then a whole new section within Spotify. And I think the decision to only update it ever Monday was pretty genius. Most people aren't particularly excited when Monday rolls around.. but when they think about the fact that it's Monday they are likely to remember they have a brand new Discover Weekly playlist to listen to. It's one of the good things about their Monday and becomes a habit over time.
I have Android Auto in my car, and my favorite part of the terrible, traffic-ridden commute from Redmond to Bellevue is listening to my Discover Weekly playlist. Even though I only really like about 10% of what it picks, usually in that 10% I find an artist I really enjoy and dig into that.
> I think it's because people are used to listening to their Playlists.
I wonder if there's any data on how common this is. I listen to large shared playlists or the radio feature the vast majority of the time to try to find new music.