It was really tempting for Ding to try to exchange just the rooks, because then it's a super easy draw if the bishops can stay on the board.
I can only imagine that Ding did not consider that black would trade rooks, leading to an oversight of the fact that the bishops can be forced off as well. So he spent all his time thinking about all the other things black can try, and didn't consider the obvious.
But with Gukesh able to forcibly exchange the bishops off too, it's fairly easy for a good player to calculate whether the king and pawn ending is an easy win or not. And in this case it was winning.
There probably is no eli5 way to explain why the resulting K+2P vs K+P position is easily winning, but it has to do with a concept called "opposition":
https://en.wikipedia.org/wiki/Opposition_(chess)
Consider the final position of the game, after black's 58th move:
Here, it is white to move, and we say that black has the opposition. It's white's turn and he must move his king, giving way for either the black king to come around it and get in and capture the pawn on g3, or for black to be able to push his f-pawn forward and get a queen.
If it were black to move in the above position, there is no way for black to do this, and the position would be a draw. Kd5 is met by Kd3 keeping the opposition, and no forward progress is possible by black.
Back to the position with white to move. 59.Kd3 or 59.Kd2 is met by 59...f4, eventually getting a queen. 59.Ke2 is met by 59...Ke4 again keeping the opposition, while 59.Kf2 black has 59...Kd4 with a diagonal opposition. In this case, 60.Ke2 Ke4 is the same position as if 59.Ke2 Ke4 were played from the diagram, so lets only consider this:
Now any move except 60.Kf2 allows 60...Kf3 by black, and white's last pawn will fall. So after 60.Kf2 Kd3, black cannot be stopped from marching his king over to take the g3 pawn. The need for white to move his king on every move will allow black to move closer and take it. Then he will be able to get one of those pawns to become a queen.
All of these ideas are pretty well known and familiar to even average tournament players, which makes 55.Rf2 a horrible blunder at even the local club level, much less in a world championship game.