Having 2 new balls in one day cricket is a perfect recipe for disaster as each ball is not given the time to get old enough to reverse. We haven’t seen reverse swing, an integral part of the death overs, for a long time.
Therefore when the spinners, who are introduced in the middle overs, are handed the ball, they effectively bowl with a ball that is hardly 8 to 9 overs old. Even by the time a spinner finishes his spell, the ball is still only 20 overs old.
Therefore, unlike earlier era, the ball doesn't get soft, which would make it difficult for the batsmen to hit through the line like they do nowadays at the back-end of the innings. The chance of reverse swing has decreased even further with the use of two new balls.