One is the already discussed age of Freelancer. Let's face facts, Freelancer is a decade old this year. In fact this month it turned 10. Yes, it may be the only game left that is a great space sim, but it's graphics are old. Plus, we've pretty much modded it out as much as we can. FLHook will only go so far.
Second, a lot of players don't even RP as much, or they Godmod (control others characters in RP). I experienced this just last night by flying through Leeds and someone tried RPing that my character did something that I didn't control. Not only that, people constantly rule lawyer, even though they're not supposed to. If anyone sneezes they get reported for something silly. I'm not sure on that though, just from what I've experienced here over my five year stay here.
A lot of people are just bored I think, there's not any new influx of things besides ships and systems and the occasional new player station. A lot of things in Freelancer take time. It takes time to build credits to do anything fun and the act of getting it, in my opinion, is worse than dailies on World of Warcraft. That's why I smuggle with a 500 cargo hold freighter, because I find it more fun. I may not make nearly as much credits as someone in a Slave Liner (which I also have, but hate how slow it is), but at least I'm enjoying myself.
These are just opinions from my point of view and are in no way true blue reasons why Discovery is losing it's members. So don't flame me. Personally, I believe a large part of it is just that Freelancer is really becoming dated now, and even though it's modifiable...we're finally hitting the limits of what we can do with it without having the source code to go any further.