Suddenly you have someone telling you what you told them and starts wondering: did I really promise that?
(Written doesn't prevent people from trying that trick, as one particulariy brilliant but not very pleasant guy tried to tell me: "yes, <my name> that is what you wrote [in the mail] but not what you meant." However, in these cases I have a written record and I am confindent with myselves and that nasty person can bugger off.)
Not saying one should never talk to people on the phone but when one are stressed out and stretched out thin it is not high on my wishlist.