To put it simply, it's because he practically begged me to drop him. Finally realizing the damage he has done to the group he is willing to give his life to help make it right and for Lee to get away. Now if he was begging me to pull him up I honestly would have dropped him, but because he knows deep down on some level he deserves what is happening to him and is at peace with it and wants you to let him die for the group shows that he is willing to do the right thing. If the roles were reversed and it was Kenny hanging, he would not ask that of you, he would demand you pull him up even though Ben and him both carry guilt for Duck and Katjaa's death (Ben for his actions at the motor inn, Kenny for leaving Shawn to die resulting in Karma getting back to him with his son's death).
Before this episode I kinda gave Ben the three strike rule. First was not telling Lee about the deal with the bandits, second was running away from Clem leaving her to the walkers, and the third was removing the hatchet causing Brie's death. After all that I was like, "Okay this guy had enough chances, he's done", but after he speech to Kenny and to Lee at the bell tower I just couldn't let him die. He is now completely accepting of his role in multiple deaths, was willing to die because of them, and now as we walk onward to save Clem, I truly believe he is gonna step up and do whatever it takes to make it right, he no longer cares if he lives or dies at this point, his only goal left is to try and make things right and I believe in the end he will die saving Clem.