I'm a huge Firefly fan but I will admit that it wasn't until Mrs Reynolds that the show clicked for me. I think Joss said it had higher ratings than Buffy did during her first season but presumably sci fi shows require bigger budgets so they expect more.
This is likely it.
The Fox network itself expects a hell of a lot more viewers than the WB/UPN/Whatever the hell "networks" were showing Buffy/Angel for all those years in the US. The expense of a show like Firefly definitely piles up on the equation, but even a cheap show that got the ratings Firefly did would have been cancelled. It was the 98th
most watched show of the season.