Most likely they are only licensed for Classic Doctor Who rather than the post 2005 Who. They're on different license agreements I think. Big Finish have a license to produce Doctor Who audios, but they can only use characters from Classic (1st-8th) Who.
McGann's Doctor has had four or five entire 'seasons' of full cast audio adventures. The final season is about to finish, and then he goes back into the 'main range' with three plays a year for each of the participating Doctors (5th, 6th, 7th and 8th ). In the audio format they've been able to give his Doctor the stories he didn't get on TV. And they are excellent. He's a brilliant Doctor. Has had a couple of excellent companions too. One of them, Charlie Pollard (Edwardian Adventuress!) was partly the inspiration for Rose, in terms of the relationship between her and the Doc.
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