My family's newest car came with Sirius radio and a free subscription that lasted a few months. We let it lapse. Chances are, the lack of market interest is what kills subscription radio. AM and FM are free.
And the threat of having to pay for it limited my interest in exploring it any further.
I'd bite if it was around $5 per month. Alas, their lowest $10 plan is going to use my data plan on cell networks