Nope, it's mainly that San Simeon was in the middle of nowhere and Bam apparently took place in the center of a densely-populated small city.
If it had happened directly under downtown LA or San Jose or Berkeley, while it likely wouldn't have killed tens of thousands, we'd be talking about hundreds killed and a bigger economic disaster than 9/11.
People should not get overconfident. The Japanese were when looking at Loma Prieta and Northridge, prior to Kobe. They thought they were prepared and had this earthquake thing figured out, compared to those silly Americans. They didn't.
That's for sure. IIRC, most of the deaths in the Northridge quake came from the apartment building on Reseda Blvd. There was another one (or 2) at the Northridge mall.
For the magnitude of that quake, and for the density of that area, the casualty count could have been much greater. Thankfully it was not.
Which brings up a point though. Time of day, where most people are active and out and about, can affect the rate of casualties. Was the quake in Bam during daylight hours?