The reason why Taylor's music sells a lot more is because her music relates to teenage girls as opposed to Katy's music, which simply entertains them. That explains why the latter does better in terms of streaming and single sales, but not album sales. If there's anything Ariana Grande has taught us, it's that radio-friendly pop songs don't sell albums if they don't go beyond being catchy and fun to sing along to. Taylor, Ed Sheeran, and Sam Smith are all dominating right now for that exact reason-they can all make music that connects to their audience, which is the real trick to selling albums nowadays.
That is why 1989 sounds nothing like a Katy Perry album. No matter how you wanna twist it, the former is a lot more personal and relates to a lot more people than singing vague self-empowerment anthems and cutesy sex songs about blowing your boyfriend on his birthday.
