1)It was badly promoted. The promo's promised something quirky and funny. What they got was something way different.
2)They ditched the 2 hour premiere in favor of the second episode, which screwed up the viewing order.
3)They put it on Friday's, which very few people watch TV on Fridays.
4)It was way too expensive for how much little money it made.
5)It was on Fox. Rupert and his boys are ruthless, cutthroat bastards. So if something isn't a hit right of the bat then they cancel it.
6)Genre TV is a niche market that does well on cable, but usually bombs on network TV. Network TV is the viewing place of the average, American, moron. So Reality Shows, Crime-Investigation Shows, and Generic Sitcom's will always be watched before anything new a different.
7) Firefly's pitch (cowboys with spaceships) sounds completely ridiculous, and may have turned off part of the target audience at first.
8) There are quite a lot of main characters to get accustomed to, perhaps too many.
9) This is a show about a band of outlaws, and the most respectable character is a prostitute, and there are many small-minded people watching TV.