Rightfully so!
The ‘verse ain’t a happy-go-lucky place. It’s dangerous. Life is harsh, and often short, ‘specially on the Rim, where humans toil and scrape to carve out homes and lives on rough-and-tumble planets. Now, you might be thinkin’ them on the Core worlds got it easy, being they’re richer and more pretty-fied… but I reckon there’s just as much fightin’ in their fancy parlors or goverment offices as you’d see in any bar-room brawl. They just hide it under genteel manners and frilly clothes, but those that don’t get their own way can turn vicious as a Reaver in a heartbeat.
It ain’t easy, living on Serenity. Takes a lot of work to keep her flyin’, from scroungin’ parts in junkyards to takin’ any job that comes along, even if it ain’t exactly on the right side of the law. But there’s freedom in the Black, so we keep working, we keep fightin, we keep flyin’, no matter what. And that’s what makes us mighty."*
There might be a way to sand off the rough edges and make the show “family friendly,” but I’d argue that would take something away from the story as we’ve known it. The Firefly crew is struggling in a harsh environment, often doing illegal things to survive, trying not to lose their morals along the way. I’m not sure how much you can bowlderize that without diluting its impact… and at that point, it’s just not the same story, which makes a reboot kind of pointless. At that point, you might as well just make a different show.
Firefly-in-name-only?
* That isn’t an actual quote, except for the last sentence. When I tried to describe what the 'verse was like, it started to come out in Mal’s voice, so I ran with it. 
Edited to add, because I missed a point: The harshness of the 'verse around our characters makes the brighter parts stand out in sharper contrast. Kaylee’s cheerfulness, the quiet devotion between Zoe and Wash, every time Mal chooses to do the right thing even when he knows it will cost them-- all these things have a stronger impact when the world around them is harsh and bleak.
It’s not completely impossible to reboot the show successfully, I suppose. (It might even be interesting to see some of the more problematic areas of the original addressed, like the conflicting attitudes on sex work.) But the odds of pulling this off well feel as likely as lightning striking twice in the same spot… especially when Disney’s already considering such big changes in the show’s formula.
