There was a time, many, many moons ago, when the Western was a staple of American TV and cinema. That trend ended several decades ago, and has never really recovered. Joss was really pushing the envelope trying to do a Space/Western. Western anything just doesn't go over so much anymore.
And Space/Westerns are considered by some, (maybe many; not sure on this one), as one of the worst types of SF. I allowed my bias to rule me, cause once I heard Space/Western I immediately lost interest. Never even saw the show until after it was cancelled.
That's when I found out that great character writing could make even the dreaded SW a worthwhile endeavor. I often wonder how many people were like me and simply ignored it from the beginning.
And I do disagree with those who say it wasn't really a Western. There were a few episodes ripped from Western plots (Train Robbery), but there were so many Western elements sprinkled through every episode. Mal packs a six shooter for gawd's sake! We're far into the future where space travel is common. Okay I get the outer worlds didn't have the latest and greatest technology. But why not an Uzi, or other more advanced side arm. Why go way back to a six shooter? Doesn't make a lot of sense.
And the guns were just one element. Outer world characters dressed in Western clothing, right along with cowboy hats. They spoke a quasi western dialect. They rustled cattle. Etc., etc., etc.