Because it's not selling as well as they'd hoped it would. Making a quality game is one thing, meeting your publisher's expectations is another.
Yeah but that's not on the dev. If CD made a game that meets internal quality and external critical benchmarks then they've done their job. Other factors outside of development affect sales. Brian's comment deliberately avoids talking about sales and the commercial reception, so in that context Square and MS should be happy with the game it's self.
Obviously if they were talking about sales it would be a different matter entirely. But they very specifically chose not to do so.
I wish people would move on. It's already announced for other platforms, Phil already "decided" to not do these anymore, and it's obvious how this one performed. No need to beat a dead horse.
This won't be forgotten for years. You will hear and read comments about long lasting franchise damage from now until even after the third game comes out and like it!