Probably because nowadays it's easier than ever to be mean without thinking, especially online. People also aren't taught as much that it's wrong, and they might not even realize they're being mean. Bullying is easy, and very tempting sometimes.
Also, people can make careless remarks that hurt others more than the mean person realizes. People say sarcastic, rude, and flat-out terrible things in the heat of anger or for fun. They sometimes say it's just a joke, not really mean, or the other person deserves it or is too sensitive. It isn't okay.