Why? Two reasons, and the most obvious is production issues, but also Samsung might thus create a feeling of exclusivity to its AMOLED devices.
Now, the big difference between AMOLED and LCD is the technology itself. While LCD uses a light source and liquid crystals, AMOLED screens are based on organic diodes. Blacks are usually deeper on AMOLED, and thus contrast ratio is much higher, but recent developments in LCD have pretty much closed the gap otherwise in color saturation and viewing angles. On the downside, AMOLEDs tend to have a blueish tint instead of displaying perfect white, an issue not seen on LCD screens.
Would you prefer an AMOLED display (like the one in the Galaxy S III) with its punchy colors and deep blacks, or would you rather get the perfect whites of a device like the iPhone 5 and the HTC One X?