That's a very good question! Just imagine how popular Nokia would be in the US!
I did hear that Nokia reportedly decided that AT&T and other carriers didn't have its 'back', and wouldn't provide sufficient marketing support -- and cheap enough subsidized prices -- to make it worth the company's while...
It's not because of Symbian though...Yes, Symbian has received negative reviews from critics, but there are actually alot of people who rather have more functionality in their smartphones than just a good looking UI...
So in the end, its all about money.But remember new Nokia Flagship smartphones WILL become available to US carriers in about 10 months time...running the Windows phone 7 OS! And i'm really excited!