I loved the National Theatre's "Oklahoma" so much that I bought the DVD--til I saw their production, I believed it was a show that was dated beyond hope of revival--even still, it was really a film, shot on a soundstage, not a stage play--

Rapaire is right on the technical aspects--the eyes of the contemporary television audience are accustomed to production values, and you can't get those by shooting a staged play, "Live"--another technical factor is time: a live play can speed up or slow down, depending on how energetic the actors are--so it's easier to edit it to the length you want, than to flash hand signs to actors--

The main problem with plays is that, by their nature, they demand more from the audience--for most people, TV is a neutral medium, meaning that you don't have to "engage" to watch it--so plays aren't going to be something that a much of the audience is going to want to deal with--