User blog comment:Cristina7/Introducing Discussions, a New Platform For Engaging Your Community/@comment-5275700-20161001024821/@comment-5288784-20161001150946

Because maintaining two different features that serve similar function would be ineffective. And a significant portion of people would use it: old abandoned wikis, conservatists, I-don't-cares, etc.

It not only will save work, but also encourages more feedback, because the reasonable-thinking people can already see that they wouldn't be ever able to just say "meh, we're staying with the forums" and need to make it as good (or as little bad) as possible when the inevitable happens.