Product Marketing Manager Bill Trinen told Polygon the reasons behind the decision:
None of the launch-window titles support dual GamePad play. As a result we won’t be selling the GamePad as a separate accessory. It will be released once the content is there to support it. Our anticipation is that there will be (games that support it) some time next year.
He did mention that once the support is added over here, they will release a second controller, so it’s just a solid money saving decision from Nintendo.
Over in Japan however, you’ll be able to pick up an extra gamepad for about 13,400 Yen or roughly $172.
Source: The Verge