Well, simply put, they chose to follow the USCF "Article 14: The Drawn Game rule 14E: Insufficient material to win on time, 14E3: King and two knights."
While it is not a forced mate, there is a mating position that is possible, thus they could have easily followed the FIDE rule, and allowed the side with the knights to continue playing.
It was probably a judgment call, and one that was made so more players would not be put off by having to play easily drawn positions for 50 moves.
I do not know if it is coincidence that you asked this today, but in the 2019 World Blitz Championship just this morning, there was a big to-do concerning Magnus Carlsen and Alireza Firouzja.
The game was up and down, but then Alireza Firouzja had a winning advantage, but in the final position below, the position was theoretically drawn when Alireza Firouzja knocked a piece over, and while resetting it, he flagged. Should it be a draw since Magnus had no pawns, or a win because there is a possible mating position? Well, after an appeal, and the rules were clear, the game was awarded to Magnus.
Here is the final position, and I will show a sample line, no matter how far fetched, that shows the rule in action. THESE MOVES WERE NOT PLAYED.
[FEN "8/4k3/4P3/3B2P1/4P1K1/8/3b4/8 w - - 0 1"]
1. Kh5 Bc3 2. Kh6 Bd2 3. Kh7 Bxg5 4. Kh8 Kf8 5. e7+ Bxe7 6. Bg8 Bg5 7. Bh7 Bf6#
The fact that this is even possible, no matter how unlikely, is why Alireza Firouzja lost under the rules. Although this is an unfair example of the rules working badly in a given position, they have to be consistent, and there may be other positions that would not be so unfair. it would be hard in practice to say, "this is OK here, but not there". They have existed in the past, but rules for different positions (like extending the 50-move rule to 100 moves for certain positions) have never worked well.