The reason is very simple, We have several plugins in this repo and several more plugins offered exclusively on our own site. In order for us to provide the best support possible we needed to find a way to keep it all in one place and have the ability to control the notifications so nothing gets missed. We have that in our ticket system but not here in the forums.
That being said, you can certainly post a question here on the forum but we cannot guarantee that we will see it.
Thanks again for using Nike+ for WordPress and feel free to contact us at the link above for any assistance you may need.