Why an embargo on Cuba but not China?

Here's a nice, little explanation of it: http://en.wikipedia.org/wiki/United_States_embargo_against_Cuba

The original reason for the embargo in 1962 was because the new Cuban government nationalized businesses in Cuba without compensation. Many US business lost their holdings there and got nothing in return. As to why that embargo continues fifty-some years later, look to Florida and its many voters of Cuban nationality.

Starling, Pete's explanation is perfect I couldn't say it better myself.
If you look at the Forbes 500 of wealthiest people you'll notice many Hispanic names. Most of them are rich Cuban transplants who live in the US. They still claim holdings in Cuba that were taken by the government.
Their money and influence has helped dictate the Anti-Cuban policy. Some have even suggested that the Clinton Administration's policies toward Cuba (Elian Gonzalez and stopping Cuban refugees from Cuba) affected the 2000 election when Miami-Dade County didn't gather as much Democratic support as in past years.
Many, including some on this site, have suggested that the Iraq war was a practice toward an invasion of Iran. I don't personally believe it. However if Iraq was a practice for an invasion of another country I think Cuba is a prime candidate for many reasons. 1. It is a continuation of the Monroe Doctrine. 2. US businesses have much more to gain (Spring break in Cuba is more likely than trips to Tehran) 3. Cuban Insurgents would have a lot of private financial backing from Cuban transplants (imagine what the Iraq war would be if Haliburton was giving money to Iraq instead of profiting from it)
Either way the next 10 years of US/Cuban diplomacy appears to be very interesting.
MikeyA