Can anyone shed any light on why Wolfram would want to create its own Community Q&A forum after witnessing the remarkable growth and success of Mathematica.SE?
perhaps because the remarkable growth and success of Mathematica.SE opened their eyes to the benefits of having a dedicated online community that can serve as a poor man's "Premier support"?
perhaps because Wolfram Research has several other products besides Mathematica, that are either off-topic here or grossly underrepresented? For example, gridMathematica, webMathematica, Wolfram Alpha, System Modeler, etc.
perhaps because Mathematica is after all their flagship product and they would like an official community/channel that they have control over?
perhaps because there are several kinds of questions that are off-topic here or not allowed? For example, discussions/suggestions/code requests/etc.
perhaps because they can?
or perhaps it's part of the next book: "A New Kind of Community" that discusses how online communities were entirely envisioned by Stephen Wolfram and how simple site rules lead to complex user behaviour?
You seem disappointed that they created their own Q&A forum. I tend to think that competition is good and that we should embrace Wolfram Community's presence and if you can afford to participate in both places, you should.
Mathematica Stack Exchange is really good at helping people solve focused questions and the goal is to create a repository of information. The community here actively closes discussions/overly broad/open ended/gimme-da-codez/etc. questions. Wolfram Community, on the other hand, allows discussion style questions where you can discuss suggestions/feature requests for future versions or use the community as a bouncing board for ideas, etc. Several WRI employees also participate in them, which is always a good thing.
Whatever be their reason for creating it, I don't see why both sites cannot co-exist and carve out their own space in the online world.
Are there features that they plan on offering that cannot be provided through Mma.SE, e.g. CDF-style live evaluation of Mma code?
We have no way of knowing what their plans are and you should ask this on their site. However, I'm fairly certain that Online Mathematica will be integrated into Wolfram Community at some point, which will allow users to evaluate code right in their browser.