Can someone who prefers reading in landscape mode explain to me why they prefer those long lines? Don't get me wrong, I'm not criticizing you, I just don't understand. Researchers tell us that a line length of approximately 8 words is optimal for reading comfort and speed. That is one reason e.g. why newspapers use columns. For ereaders that means portrait (unless you use very large fonts).
Back on topic: some e-reader manufacturers may have read about this research as well, and decided that it's not worth it to implement landscape mode.