Because we're mid-console generation. All serious console gamers already have at least one current gen console, so we aren't going to buy a new one for any reason but exclusive games or unique features. And even if someone were only concerned with sheer horsepower, they'd just wait for Scorpio, because I doubt this thing is beating it.
Yeah, third party support is important, but it's also expected and taken for granted. Nintendo hasn't had it simply because they've been an entire generation behind their competitors. In the end, exclusives are the actual selling point of any new platform.
Huh? It's already confirmed for Xbox and PS4.