different pricing, roads, driving styles, tastes etc...
1,200 vettes total for europe in 2007 is even lower than I thought. the only US car I see sometimes in the UK is the 300C (which is on an old merc chassis iirc), cool looking car 
edit: btw in 2007 porsche sold ~13,000 911s in the US.
Yup the US has wide roads and lots of room. Heck just driving across Texas is the equivilent of driving through several countries in europe.
A friend of mine had visitors from Germany. He took them to San Antonio for the weekend to visit Sea World. They had been driving for about 3 hours when one of his friends spoke up and asked if they were still in Texas. He answered yes, we are only going half way across Texas to get to our hotel. They were amazed at all the open land and cattle. They said they now understood why we have cupholders in our vehicles. lol