If you look through their releases, the growth appears to be coming from doing a better job of getting a free-trial included in almost every new car and like half of used cars (I assume this is done through dealer networks).

Given that, if free-trials convert at a steady rate, voila, you have growth.

In terms of why people like it:

1) There's something somewhat Tik-Tik-like about having hyper-specialized music served to you without putting in the work of building a playlist

2) Streaming services have the somewhat annoying tendency of 'recommending' new songs to you so they can pay the artists less, since they were a 'discovery' mechanism. This leads to bad outcomes if all you want is 80's on 8 (said differently, if you like 80's music, chances are you know the songs you like already)

3) The lowest their is $13/month, which is cheaper than a phone plan with more data for a lot of people

4) There's a certain 'Sunday Ticket/League Pass' element, since you can get the radio coverage of nearly any pro sports game (even in home markets I often struggle to find the radio coverage of certain games)

5) For the traditional satellite service, they're broadcast only so they have no idea what any subscriber is listening to at any point, and some people like that.