Pandora is a music discovery tool that is not based on popularity. That is, if you tell Pandora you like a certain type of music, Pandora is about as likely to play similar obscure music as it is to play similar popular music. Other similar services such as Spotify or Last.fm base their music suggestions on the person <-> song graph rather than on the attributes of the music, so they will primarily suggest very popular music (which I will flagrantly assume is well correlated with Amazon sales rankings).
Edit: Oh, I see. I assume these artists just have much better music than they have marketing. It would be interesting to know what sets them apart, though.
Right, I get what the differences in the models are between Pandora and other services. I'm referring to how the artists and/or the music itself differs that causes them to fit into this strange category.
The number of artists in Pandora's catalog is very small relative to all other mainstream services (because of the cost burden of onboarding artists into the Music Genome Project, a manual process). So, put an unknown artist that sounds like Coldplay into rotation and every user who starts a Coldplay channel is going to get them a bunch of times. Do an unknown band that sounds like Guns N Roses, same thing.
Discovery, I'd imagine. Amazon has a "you might like this" but most of the time clicking those links is not actively what you are doing on Amazon, whereas it is Pandora's whole model.
