It happens.
In my opinion, a lot of users of streaming media services are neither enthusiastic customers nor entirely forgetful. Their opinions of the services they pay for aren't consistent from day to day, either.
Spotify was never going to have me as a subscriber but Apple Music was so much better (for me) that it was a no-brainer.
But then I always preferred Pandora over Spotify anyhow (different use cases I know, but still).
1. I can add music to my streaming library that is not on their service and that music isn't treated any differently than the stuff in their catalog from a UI standpoint.
2. It integrates better into the Apple ecosystem, which I've already tied myself too. For example, play music using Siri and Apple TV support.
Also, I'm not a fan of Spotify's UI.
2. Spotify works great with Google Now and Chromecast.
Also, I'm not a fan of Apple Music's UI.
I know Apple Music gets hate for it's UI but it's nirvana compared to the horrible, grating, death by 1000 papercuts UI that Spotify has.
So that contributes a LOT to my impression of Spotify.
I find their radio offerings better than Spotify, and the curated playlists to be of higher quality and better variation. The For You tab alone is better than a lot of what I found on Spotify.
The importing of my existing library and linking of it into the service with all sorts of metadata made for a seamless experience that I haven't found from any other service either.
Basically... it's just a little bit better (for me) in every possible way, while being nicer to use.