A major reason for the enduring use of satellite in radio distribution is that, for live events like sports or (more common in NPR's case) political events, the satellite system provides appreciably lower latency than distribution over the internet. Reduced jitter also allows for generally higher reliability, you never hear the radio station buffering. There are options for low-latency land-based connectivity but at the scale of PRSS, the satellite system is cheaper to operate.

Most stations can also receive this programming over the internet, another reason for the satellite system is that it provides a completely redundant path for programming delivery. This is important for general reliability but especially so in an emergency.

Historically, radio networks distributed their programming over leased telephone lines. Satellite took over because it was cheaper. That gap has probably narrowed as terrestrial communications infrastructure continues to expand, but the internet struggles with low-latency real-time media, and an arrangement like leased fiber wavelengths to member stations would still be more expensive than the satellite system. There's a lot of member stations in a lot of places, satellite reaches all of them at once.