Because every sentient being deserves compassion and care. Because no sentient being should have to suffer. Because their unique experience, thoughts, perspective, and outlook enriches us all. Some of humanity's greatest minds, scientists, advancements, art, and philosophy, have come from people who have lived in society's margins.
If this is difficult to understand, you can try a few things.
First, try putting yourself in this person's shoes. Do you feel like you should have to justify to someone why you should not have to suffer? Does it feel good to be made to feel that way? Do you deserve to feel bad? The answer to all of these is, "no". It is the birthright of every person to have the best life possible. Full stop.
If it's difficult to put yourself in someone else's shoes, you can try thinking of yourself. People who are hurt and have had bad things happen to them often can't help other people. They can't contribute to humanity to the best of their ability. Sometimes, people who are hurt, reflect that pain outward and hurt other people. All things considered, in the statistical soup of the world, if there are fewer people who are hurting, you're more likely to have positive experiences. In Buddhism we call that karma.
“I am, somehow, less interested in the weight and convolutions of Einstein’s brain than in the near certainty that people of equal talent have lived and died in cotton fields and sweatshops.” - Stephen Gould
Still here, but I check infrequently.