The Giants defense had played the same defense most of the game
The Giants didn’t offer a lot of variety throughout the game defensively. Their idea of limiting the after-catch explosive potential of the Rams offense was to keep the offense in front of them. The Giants have very successfully switched to a Tampa Cover Two scheme. The philosophy of the Tampa Cover Two is to play a lot of zone defense across the board, two deep safeties, and lots of underneath defenders.
You see, what the Tampa Cover Two allows for is a lot of flexibility. The Giants were dropping different players into different zones across the formation just to keep Goff guessing for the majority of the game. It allows defenders to read the play quickly by limiting their reads and responsibilities on the play, and a well-coached unit plays faster and more aggressive as a result.
Black belt Rams
The Giants are indeed well-coached and indeed played faster and more aggressively. The Rams simply used that to their own advantage in this play.
This time, the Giants mixed up their play call. They went with a heavy look at the line of scrimmage to discourage a quick run, and then dropped the inside linebackers. The pressure came heavily from the left side of the line, trying to load the cornerback with a free look at the quarterback. By sending three defenders at two gaps, they hoped to overwhelm that side of the line and render the right side as useless.
Notice also the number of defenders playing underneath coverage. There are five guys all playing underneath. This is why the Rams struggled all game to move the football, there was a wall of defenders waiting for them to throw short passes over the middle.