It's because you don't have anything left in the tank to take yourself over the top.

The other guy is beating you down and you have nothing left. 

The Pats had the ball on the 50 with just over 2 minutes left. All they needed was one first down. The O-line disappeared on that series - the most important series in the game.

Then the D had their chances. Make a stand and force a punt or a 4th down conversion attempt anyway. 

The refs were terrible and it was a tough away game against a tough team but the Pats had those 2 clear chances in the last 2 minutes and were too pooped to do anything with them.