Thanks for the replys. How about answer it like this?

I have been working for my current employer for ** years. I would like to go to a new evironment to learn something new, to take new challangeds so that I can grow my career further and faster. I believe that your company will provide me a better oppotunity to achieve this, espcially, as you might heard of, that my company is shifting its focuses and the area that I am good at is not in the core focuses any more.