This is a 2 parter. First part will be the rule enforcing the call, second will be the reason behind the rule.
There is a special rule surrounding fumbles after the 2 minute warning of either half. Article 6. Fumble After Two-Minute Warning of the NFL rulebook states:
If a fumble by either team occurs after the two-minute warning:
a) The ball may be advanced by any opponent.
b) The player who fumbled is the only player of his team who is permitted to recover and advance the ball.
c) If the recovery or catch is by a teammate of the player who fumbled, the ball is dead, and the spot of the next snap is the spot of the fumble, or the spot of the recovery if the spot of the recovery is behind the spot of the fumble.
Note: The restrictions in (b) and (c) are applicable during a Try throughout the game.
So when Kareem Hunt recovered the ball, the ball was called dead since he was not the original fumbler.
[Second Part] Now we get to the more interesting question, why is there such a rule? The answer is the The Holly Roller, you can see the play here:
The 1978 Raiders fumbled their way to victory in a game trailing by six and 10 second left. Their QB Ken Stabler intentionally fumbled the ball right before getting sacked. You can see #40 also intentionally throwing the ball towards the endzone instead of taking the possession where it ends up getting scored by Raiders and giving them the win. Although the play was legal, it was clearly a loophole in the NFL rules, which was patched by the above rule in that immediate off season.