Because it effects how we interact with people in this life. Most people view original sin as a dark and depressing topic which Christianity would be much better off with, but when you truly understand original sin, and you truly understand Pelagianism, one has noble implications for how we react to others, and the other has worrisome implications.
If humans are naturally inclined towards sin, and are incapable of being faultless, in not only becomes proper but purely reasonable to forgive others transgressions against you. Being insulted, assaulted, threatened, etc. is a natural part of interacting with humans, so you cannot hate others for it, especially since you do it yourself. We turn the other cheek, because after all, the person sinning is only human.
By comparison, Pelagianism means everyone is accountable for every moral decision they make. If someone strikes you, it is because they made a deliberate choice towards evil. Now at that point, the rational aspects of the doctrine of universal love and forgiveness start to break down.