Most of my stations are instrumental because I tend to prefer music without poetry. It seems to be challenging for many genres to produce engaging instrumental music because they rely on the changing poetic content to retain interest. This is why simply playing a vocal part on another instrument is boring in exactly the same way that a song where vocals repeating the same words over and over is boring. Composing a song from the beginning as an instrumental piece is sure to help.
This is less of an issue in jazz. Instruments trading solos helps keep things interesting (improvisation helps too!). I imagine that Classical music avoids this somehow, though I tend to find it uninteresting for reasons that are probably unrelated.
Though there aren't many strictly instrumental blues bands, the blues does have a rich history of instrumental songs. This means that where it's easy to seed an instrumental jazz station with artists, instrumental blues is almost entirely seeded by specific songs.