300k sold for a top hyped EXCLUSIVE from an established studio like Insominiac.....very bad. Considering there were approximately 6 million Xbox1 install base, that means there was a .05% attach rate for the game, with 300K copies sold. That is not good by any standards. Usually it's at least 1% attach rate. 5% or 10% is considered a success. .05% is a failure.
That's why they are giving the game away for free for a day, in an effort to show people that it's a good game, and to buy it. Problem is, people that buy an Xbox just aren't interested. Xbox only users are primarily FPS fans primarily, and look for the dark grey/green/brown hyper-realistic games. SunsetOverdrive is a 3rd person action/shooter with bright colors and that's the opposite of what the base of Xbox fans look for.
Insomniac games aren't huge sellers. Maybe you should look at the launches of the Ratchet and Resistance games.
Sunset Overdrive is in between those and on a much lower userbase than Resistance 2 and 3 were. On top of that it's a new IP as well.
Also as already pointed out, the attach rate is much higher than 0.05%. It's 4.5% (week ending November 8th)