What's the rationale behind the choice of the current User Content License?

So I had another look at the terms of service today - the first time after registering.

Under the section “User Content License”, it currently states

User contributions are licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License.

I think specifying the license is useful, but I do not see why the license is not an open one. The problem is the NonCommercial clause here — a Creative Commons Attribution-ShareAlike 3.0 License is used on Wikipedia and StackOverflow, and I think it would be appropriate here as well.

I would also be fine with something more open — my own contributions to anything public are in the public domain/ under the CC0 1.0 Universal Public Domain Dedication (CC0 1.0). For the purpose of this ticket, I just want to get a deeper understanding as to why the current arrangement was made, and whether the chosen license is the most appropriate in this context.