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Because apps can be developed to rotate themselves, or even have a completely different look in each orientation. Take Mail for example, in portrait you only see a single message, in landscape you see a message with a subject list on the left hand side.
The iPad does not rotate your background (or lock screen) graphics when you swing it around, the image is fixed in place, and the viewport slides around. If you have a graphic that has an orientation (a building or people for example) it might look funny when it is on its side. Best to have generic images or patterns.
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