Not being able to see the characters doesn't stop audio drama being popular. The
longest-running soap opera in the world is on the radio, and it has possibly the most fanatical fans of any soap opera even though they've never seen any of the characters. And I suppose if you're talking about science fiction,
The Hitch-Hiker's Guide to the Galaxy was originally a radio series, and that radio series is still superior to the film, the books, and the TV series (just). The comparison is more appropriate than you might think - I've only listened to a couple of the Dr Who audio episodes (with Paul McGann), when they were on the radio, and they were quite funny ones - not unlike Douglas Adams.