I honestly can't say I'm surprised by this. I love the Firefly TV show and Serenity movie and I would love to see the ship rendered in LEGO. However, I understand that a process of this size and scope would need to appeal to a wide audience, which I highly doubt it would. The show is for a niche audience and the limited fanbase is one of the reasons it was cancelled and why the movie was pretty much a financial flop. Add in the violent content and the pseudo-zombie enemies, and you can see why LEGO would want to steer clear of this property.
Here's the note the CUUSOO team attached to the project:
When reviewing projects for production, the LEGO Jury first looks at how the concept fits the LEGO brand. We learned the IP was questionable regarding a brand fit, and since we anticipated this project reaching 10,000 the team got a head start and reviewed the project on these criteria.
LEGO produces toys for children. Therefore all LEGO products, regardless of age target, must be content-appropriate for this core audience. With this in mind we have decided that as cool as the Serenity model is, the Firefly TV show and Serenity film contain content that is not appropriate for our core target audience of children ages 6-11. While we know this news will disappoint those who supported the project, we will not be producing this as a LEGO product.
Opening ourselves to new product suggestions invites popular ideas that don’t always fit our brand. We are grateful for the spirit behind projects like the Firefly Serenity and for the opportunity to be challenged. It keeps us sharp and looking toward the future of the LEGO brick.
This basically shows the team is starting to give potential themes much more thought. While they may let voting run its course to attract people to the CUUSOO idea, they are trying to make it clear that support does not equal production.
I guess if you want your own Serenity, it's probably best left to a MOC project.