I have an iPhone/iPad App on sale that is designed to work only in portrait. The reason is, for best aesthetics, it has a row of buttons at the top, and a row of buttons at the bottom, with an interactive zone in between portraying 3d graphics that for functional reasons, has to be exactly square.
If I allowed rotation to landscape, especially on iPhone, after the loss of space caused by top & bottom button bars, the interactive central square would shrink too much and cease to be appealing. So rather than redesign the whole UI with buttons in different places, I simply disallow landscape. I have had various feedback, and requests for different features, but nobody has ever asked for a landscape version.
I have other Apps that support both orientations, where doing so is appropriate. When I publish an App that does support both orientations, I ensure the App Store screenshots illustrate them both. But if my screenshots show only a single orientation, I assume purchasers will figure out that is the only orientation they should expect.