There are many factors for an answer to your question.
Since I personally experienced the San Francisco Loma Prieta earthquake in 1989... let me try to answer this for you.

First off, the '89 earthquake was a 6.9 on the richter scale. I know that's not much of a difference, but every point counts when measuring energy. Also this earthquake only killed 63, but it injured almost 4,000 people.

Another big reason for the difference in casualties has to do with the very strict building codes that all structures have along the fault line in California. No brick structures are allowed, and only grandfathered in with drastic updates. The structures in Haiti and other 3rd world countries that don't have strict building codes suffer a much higher casualty loss.

If you also factor in the time of day, you can have many more casualties.
The earthquake in China that killed so many had the worst time of day along with unstable buildings to add to their outrageous death toll.
http://www.guardian.co.uk/world/2009/may/07/china-earthquake-anniversary-death-toll

The time of the SF '89 quake was a few minutes after 5:00 pm on a day that the entire Bay Area was getting ready for the Battle of the Bay. Both baseball teams were playing against each other in the World Series (Giant's vs. A's). Many people took time off that day or were already at Candlestick stadium which is in an open area.

I was one of those people. I had tickets to the World Series and was outside when the earthquake happened. I also think that the game saved many peoples lives. It saved mine. I usually stay at work until 5:30 everyday and my desk was sliced in half from the huge filing cabinets that fell down on it.

People were also astonished at how light the traffic was on that day, again... because of the game. If it were just a typical day with the collapse of the Oakland freeway, we would be looking at numbers in the hundreds, maybe even thousands.

So, as you can see... many factors take place when an earthquake hits. Time of day, type of structures, and also the amount of aftershocks can play an important roll in further destruction.
San Francisco is much more prepared for an earthquake and the residents understand that, because it's not a matter of 'if', it's a matter of 'when'. With poverty stricken Haiti, you can prepare or plan for an earthquake when everyday is struggle in survival. My heart goes out to all of them.