How come if someone tells about their medical condition, someone might say "that sucks". that's so rude, i would never say it sucks i would say sorry?

Different styles: I can't answer for those who say that but i would hope they are not trying to be rude. Hearing of someone's problems and saying "that sucks" is another way of expressing sympathy. While it might upset the sick person, some sick people would understand the sentiment and not be offended. Different people have different styles of communication and some are more sensitive and refined than others.