Why does the U.S. elect so few scientists ?

Excerpts from an op-ed piece (by a mathematician) in the New York Times:

I’ve visited Singapore a few times in recent years and been impressed with its wealth and modernity. I was also quite aware of its world-leading programs in mathematics education and naturally noted that one of the candidates for president was Tony Tan, who has a Ph.D. in applied mathematics. Tan won the very close election and joined the government of Prime Minister Lee Hsien Loong, who also has a degree in mathematics.

China has even more scientists in key positions in the government. President Hu Jintao was trained as a hydraulic engineer and Premier Wen Jiabao as a geomechanical engineer. In fact, eight out of the nine top government officials in China have scientific backgrounds. There is a scattering of scientist-politicians in high government positions in other countries as well. German Chancellor Angela Merkel has a doctorate in physical chemistry, and, going back a bit, Margaret Thatcher earned a degree in chemistry.

One needn’t endorse the politics of these people or countries to feel that given the complexities of an ever more technologically sophisticated world, the United States could benefit from the participation and example of more scientists in government...

Among the 435 members of the House, for example, there are one physicist, one chemist, one microbiologist, six engineers and nearly two dozen representatives with medical training...

I wonder to what extent the situation is similar or different in other countries. The article doesn't offer answers to the question posed, but some of the ~400 comments there are particularly insightful.