Having Cuba shoot down two of your nation's civilian planes tends to mess up relations.
Sure, but there have been far worse incidents between the U.S. and other countries which have not lead to trade-embargos. Besides, even that happened six years after the collapse of the Soviet Union, and nearly 25 years after ties with China were normalised.
The China bit is irrelevant considering one nation's relations with the United States can't be used on a basis with another, when Cuba and China are so radically different. Not to mention..... the planes being shot down did not start the trade-embargos - just strengthened the ones already in place, which I find acceptable considering the incident. What are some of these "worse incidents" if you don't mind me asking?
Well, for example:
http://en.wikipedia.org/wiki/Korean_Air_Lines_Flight_007
62 American citizens died in this single attack, but the United States never broke off diplomatic relations. I'm not saying all these situations are analogous, but the reality is that the U.S. is allied to far shadier nations than Cuba, with which it has no diplomatic relations and and a 50-year trade embargo.
We have no permanent allies,
we have no permanent enemies,
we only have permanent interests.
–attributed to Henry John Temple Viscount Lord Palmerston 1784-1865, Foreign Secretary and
two-time Prime Minister under Queen Victoria.