Why is it?
Take the fantastic Alex Rider series, written by Anthony Horowitz, or the awesome Jason Steed series written by Mark A. Cooper, (Although he lives in Florida) and Jimmy Coates written by Joe Craig or Young James Bond written by Charlie Higson and finally the CHERUB series and Hendersons Boys series both written by the crazy Robert Muchamore?
Oh, well I did forget the new Spy School series, written by Stuart Gibbs, he is American.
I never thought of it until you asked. You also forgot Enid Blyton, she wrote Famous Five and Secret Seven, she was also British and if you have read Mark A. Coopers series Jason Steed has a similar style. (Simple straight forward POV)
We need some American authors to write teen spy books.