I think this is not a simple question to answer. It may be more than one thing.
But when I look at both teams last night the biggest difference to me was the Packers made very few mistakes all night and when they made them, the Cowboys did not get fired up and take advantage. The Cowboys made a ton of mistakes all night. The Packers took advantage of almost every one.
The Packers played as a team, especially on defense. Every time Dak hit a receiver with a pass it looked like 10 Packers were there to make the tackle. That's how defenses are supposed to play. The Cowboys played as 11 individuals and that is why the Packers had an easy time moving the ball. There was no better example than the play where Bell and Wilson both left the TE who them ran uncovered downfield. Love threw a balloon up in the air of his back foot to him but no one was anywhere near him to knock it away or intercept it. The receiver had to wait for the ball, and still casually strolled into the end zone before 1 guy, Kearse came over way too late to challenge him. How does that happen 18 games into the season?
Mental mistakes, little and big, is why Dallas lost that game last night. They are a better team than Green Bay, and I suspect the 49ers will show every just how mediocre the Packers really are.
I blame Quinn for not have his unit ready to play as a team. He had a terrible game plan and it was poorly executed. I have been saying for weeks that Quinn is not the guy we thought he was and last night, I think he proved it.
My advice to Dallas, though, is stop looking for certain types of athletes of certain sizes or certain arm length. Draft football players who have big hearts and know how to play team football. The defense needs a hard hitting LB with good instincts, and the will to win and elevate the entire defense. They do not have enough of those guys on this team.