Unions, I believe, get the biggest blame of the options given. As someone pointed out, no company can long survive when you're payroll to retirees is greater than that of those who are actually working to produce the product.
Total salary and benefit packages get expensive. According to 2006 data, US autoworkers make, on average, $73/hour (the average worker in the US makes under $30/hour). That's GREAT money, but that cost is paid by the consumer. "Import" autoworkers in the US, on average, made about $44/hour. On average, a vehicle made by the UAW has an additional $1000 on the sticker compared to non-union automakers. So if you buy a $14K car, you'll pay about 7% more for a US car than an import simply to cover union benefits.
I do believe that auto makers themselves have to share some responsibility. I believe the quality of US cars began to go down when the accountants began deciding which cars could be build rather than designers and engineers. The bean-counters looked at production costs and seemed to ignore market trends. Automakers churning out SUVs was a GOOD move because that's what people wanted. But that hit a brick wall because of my next point.
Another share of the blame also must go to bureaucrats. Imposing heavy regulation forced automakers to spend tons of cash to meet guidelines that may not be a response to consumer demand. Gas hitting $4/gallon was a big turning point but, again, the responsibility for that lies with bureaucrats that essentially forced the price increases by not allowing oil to be drilled domestically. Also, newly proposed carbon tax legislation will add around another $1300 to the price of a vehicle. Simply put, legislation can make cars far more expensive to buy.
So were overseas manufacturers really determined to give US customers what they want by building smaller cars? Not necessarily. While the US is a very lucrative market to them, they also build cars to meet the demands of buyers in other countries where small cars, even micro cars, have always been the norm. Plus you can't ignore that gas also costs $6/gallon over there. People in the US tend to want bigger cars that hold more stuff and that have more power.
Basically, I believe it's been a "perfect storm" of sorts that has crippled the US automakers. A combination of bad decision making on the part of the manufacturer, burdensome legislation and tinkering with the free market on the part of the bureaucrats, plus astronomical costs demanded by workers and retirees. It was only a matter of time before the bottom fell out.