Why don't we have a key board which has all the letters in a alphabetical order?

I believe that one reason this will not happen any time soon is that it would be a very hard transition. Its basically like learning a new language. You might want to say "Hello" but instead you type "Pcssi" instead with an "ABCDEF" keyboard. It might make more sense, but until you can change your keyboard layout without changing your entire keyboard, I doubt this will ever happen.