Springer
Regular
My understanding is that it is related to WB having many different competing branches. B5 was part of the PTEN branch and the show was produced independently of the WB studio (by Joe & Doug's studio); there is probably no branch in WB left to support it.
PTEN dissolved before season 5 was even filmed. There was quite a bit of merchandising after that occurred, as well as spinoffs. I'd have to see something concrete to believe your premise. Things dried up after JMS stopped playing ball with them. They know that as long as he's alive, fans won't accept anything he doesn't approve of and it's already a niche product as it is.
JMS has regularly spoken about the infighting between departments at WB. Here's a statement he made at a convention in 2013: http://freebabylon5.com/frequently-asked-questions/ (Click on the 'why isn't B5 available in reruns' question)
To quote from what JMS says... "[PTEN] that was outside of Warner Bros. Television. Warner Bros. Television is like the big TV shows and the sitcoms and the Law & Orders and all that kind of stuff. And these [PTEN] executives said, “We want to make television, too. We’re part of Warner Bros. Domestic Distribution, we want to have shows”. And these guys [Warner Bros. Television] hated those guys [PTEN]. “It’ our job to create television, not your job. You’re just syndicated, you just set things up. What the hell do you think you’re doing?” And that enmity was there from Day 1 and persisted for the entire run of the show which is why all of the PTEN shows were kind of bastard children to Warner Bros. Television, particularly us because we did well which really pissed off the guys [at Warner Bros. Television]."
Would JMS even work with Big Finish? Would Big Finish want the property without JMS approval? They do good work but where would JMS make the time to provide outlines and approve scripts? And maybe WB doesn't want to work with JMS anymore.
I agree, if JMS and the original surviving cast wasn't involved I wouldn't be interested in audio dramas. Any fiction going forward has to be canon, otherwise I can't see the point from a creative point of view.