In the end, I guess, it's that Joss tried something new, unexpected and unconventional.
Because nobody's combined space opera and westerns before. Heck, I've seen people argue that space opera is just a natural outgrowth of the western genre (I'd never go that far, mind.)
Firefly was a Joss Whedon genre mashup, I enjoyed it for what it was but it was neither new per se nor (given we're talking Whedon here) unexpected.
Okay, let's amend my statement to "new, unexpected and unconventional on American TV
." I'm thinking Fox execs never saw Outlaw Star, Cowboy Bebop, Blake's 7, or realize what Star Wars' roots are. Likewise a majority of US couch potatoes.