If you watch some of the rough cut tapes of the gang in action, there is always the discussion of wither the film or short is "too talky".
And believe you me, for fan riffers, its just a big of a problem. If its too chatty, you can't get your riffs in.
On top of all of this, Plan 9 is sort of a holy grail of bad film.. they may have thought that it was so bad that it was not worth doing.
However, I do enjoy the Rifftrax treatment.