Here are some random thoughts with no scientific research backing them up.....
Americans like German cars. I think most Americans would prefer to own a German car based upon the test drive experience only. However, some of the considerations below come into play when explaining why more German cars are not seen in the US. Many parts of the US are flooded with nice German cars while some towns have an extremely small precentage of foreign cars (regardless of origin).
German cars are substanitally more expensive to repair than German cars available in the States. These cars are also more expensive to purchase outright. The reliabilitiy of German cars is quite poor over the last several years (particularly with "systems" issues). A basic/common American car will almost certainly have better reliability than a VW
(basically the entry level German car in the US).
A fair number of US buyers are not enthusiasts. For such buyers (on paper), US cars do offer a pretty striking value when looking at space, power, etc. However, I personally would argue that a more expensive Toyota represents better long term value than a cheaper (but similarly spec'd American car)
Many people I know (who shop for American cars) have a pretty good idea of what they are going to buy before they test drive a single vehicle (many times they will simply turn in the old XYZ model for the latest XYZ model every few years). Hell, I bought 2 cars in one year without a test drive.
Some people believe that it's their duty to buy an American vehicle. I've known a few people who fought in WWII and have sworn never to knowingly buy a German or Japanese product (although that must be pretty tough these days - however, I do respect their decision). Other shoppers buy American vehicles because they believe it supports their neighbor (or shows solidarity with a union). This is something else that I'd personally dispute as I can easily buy a Toyota (or Honda) made in America by Americans (which isn't always the case with American brands as some buyers might believe).
Something else to consider is that many German cars have more standard equipment in the US than one would expect in Europe (or at least this has been the case in the past). The growing list of common "standard" equipment in the US is actually creating greater value for those who wanted the equipment anyway ("most people"). However, the other effect of this is that the baseline price will also be higher and may discourage some buyers.
Finally, the pricing games that are played with American cars are a joke. Several months ago a dealer was selling the remaining stock of (new) 2006 Ford Tarus sedans for 45% off the sticker price. Under more normal negotiation circumstances, the negotiation room on domestic cars is generally higher than on foreign cars.
Personally, I'm a huge fan of the German car driving experience but am waiting until I have more money to spend on buying a higher performance German car. Until then I have a Mazda MX
-5 and Lexus ES300 - Both quite reliable and better designed than any comprable American car. I'd never consider an American car apart from a full size truck or large SUV
(if I thought I needed either) or C6
Vette. Giving it a second thought, I'm not sure that I'll even bother (eventually) with a German car considering the maintenance issues (and associated cost of repair).
Some of my contributions are probably obvious (and parallel rationalizations could exist for a number of other products/countries) - Others are probably limited in application and reflect only things I've observed personally.
I should add that I see a ton of German cars in the US. What information are you using to form your opinion (or your suggestion of doubt) that the US doesn't like German cars?