A few reasons...
1)Mutliple BIOS manufacturers, each having multiple updaters. You would have to either brute-force them, or have the virus ascertain which updater to use. This would require some decent programming skills, a proficiency which history has shown is not exactly something the average virus writer possesses.
2)On-board BIOS virus protection. I think most boards include this, and most of them have it enabled by default. Of course they usually just ask if you want to allow the change, and I imagine most average joe sixpacks would just say yes.
3)Many BIOS updaters won't run while the OS is loaded. It's going to be a tough sell to write a virus that will reboot the PC and then somehow execute itself prior to starting Windows.
But I think the big one is:
Too destructive. Virus writers usually want to spread their handiwork. As a matter of fact, that's how the term "virus" pretty much originated - a program that replicates itself and spreads from computer to computer. They often include payloads to steal information or conduct DDoS attacks, but fundamentally, if you destroy your virus can't spread if you destroy the host PC. Sure, you can spew some copies before you destroy the host, but it's more effective to just spew as many copies as you can before someone notices and shuts it down.
