from my personal experience it is because it's hard to operate any touch screen device one handed, no matter if its landscape or portrait.

touch screens (especially capacitive ones) require you to keep your thumb away from the device until it reaches the button you want to press, only then touch it. this is really awkward if you try to do it for a while. much easier to use your other hand (which is far enough away from the device).

this is one of the reasons why for every day use, i prefer real phones with numeric keys and softkeys - like the my S60 model. you can safely keep your thumb on the device all the time, it will never cover the screen, you can actually feel when you reach the next key and there's no way you could accidentally press a key when you only want to rest your finger on it.