Spotify is horrendous. Their UI is subpar. Their mix of podcasts with songs is odd at best (seriously, I never listen to podcasts, why can't I switch it off completely?). Their shuffle is just not. Imagine listening to the same 20ish songs from a playlist of nearly 2k songs. How can you mess up shuffle that bad? It's also not easy to move away from it. It requires time and effort. Time that many don't have.
I wonder if it's just different ways that people listen to music that lead them to prefer one service over another?