Maybe because a new 64-bit Operating System might be a little buggy at first? Who knows. 32-bit OS's are still good, but a 64-bit edition would make everything much better. All your applications will run faster, plus, you'll finally be able to utilize that 64-bit CPU!
Also, why are you calling Microsoft?
[EDIT] Nevermind. I read it. Looks very professional. Nice letter. Definetly worthy of a rep point.