People aren't happy with Nintendo announcing a Circle Pad Pro XL for the 3DS XL. Some say they are "cashing in" on consumer ignorance.
But there is a very good reason why they aren't building the Circle Pad Pro into the 3DS XL. That's because the Circle Pad Pro is more than just an extra Circle Pad, it gives the 3DS four shoulder buttons for console-like gameplay while destroying the unit's portability.
This is the reason why the 3DS XL doesn't include the Circle Pad Pro.