The programming reflects the Opry. Yes, the Opry has a lot of big name members. But they also feature a lot of no-names. At one time, WSM was a traditional classic country station that played country hits from the 60s and 70s. Then it evolved into a classic Americana station with a variety of "American" music. So it's not really a radio format, but a music format. It's what radio would sound like if it was run by musicians rather than radio people. Which is in a word, a mess.
Here's who is on the Opry Saturday night: T. Graham Brown, Gary Mule Deer, Tom Douglas, Riders In The Sky, Matt Schuster, Brittney Spencer, Rhonda Vincent, and Dee White. You're likely to hear their music on WSM. There probably is an audience for that. People pay to see them at the Opry. But it's not a large audience, and the station has a better chance of reaching them online.