From what I recall, Pandora limited themselves to radio (license) rules, which is less appealing as interactive. Interactive requires specific licensing deals,

From what I recall, Pandora limited themselves to radio (license) rules, which is less appealing as interactive. Interactive requires specific licensing deals, which will include minimum annual fees in the millions. Further, if you try to launch in the US, you're essentially "in the eye of Sauron" with music labels and subject to tons of microaggressions, as well as the Apple 30% tax. By launching Spotify only in the EU, and launching early, Spotify wasn't exposed to so many hurdles. They gained a strong enough foothold that allowed them to pivot to the US safely. Their entry forced Pandora to pivot to interactive, and triggered Apple's attempt to Sherlock them with Apple Music. Fortunately, Spotify was too big to get squeezed down.