Short answer: Because, due to corporate in-fighting at Warner Bros., they simply don’t make it available.
Long answer: From the ‘Fandom Endures’ panel at Phoenix Comicon on May 25, 2013:
JMS: “The problem we have currently…it’s still on the air overseas. It still airs in the UK, in Australia, Germany, France, everywhere else but the US. And the problem is, even with availability online or on DVDs is that you have to go looking for it. Think about it. Where Star Trek took off in syndication, you might have been flipping through channels and you’d happen to land on a channel where Kirk’s fighting a Gorn and you think, “What the hell was that?!” and you stick around and watch the episode, and gradually you become acclimated to it. The only way to find the show in the US is to go looking for it.
The reason for that is petty beyond belief. When we started out, there was the PTEN network that was our first home on television syndication. And 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].
When we went off the air finally, they couldn’t have been happier. They were looking for five years to shut us down. They seriously tried to sabotage the show numerous times but they couldn’t pull it off because the fan support was there. And once we were done, they scooped it and said “This is not part of Warner Bros., we’re not concerned about it. Put it online to help drive other platforms, to drive AOL or drive the online [content?], or drive Warner Bros. network online. But to actually acknowledge their own show, that it had any integrity or importance? Fuck no. And those people are still there dragging their heels.
It’s my abiding hope that over time the fans will gather up and say, like ‘Free Mars’, free Babylon 5. And start the fan movement to put it back on the air again. That may have a long term effect of making them aware of it. Because it’s now been put aside by the guys in charge being “below our notice”. It makes money for them. You know it made half a billion dollars in DVD sales, of which I’ve never seen a penny, nor has most of the cast.