I think screen real-estate could be a problem, as typing large amounts of text in landscape view can make it difficult to find where you are in the text as there’s not much room for that to be displayed when the keyboard is in use.
But for other apps landscape would be nice, even if it meant putting some controls at the ‘side’ when in landscape mode. Though Apple do like consistent design so I doubt they’d put buttons at the side instead of top and bottom.