Speaking as someone who worked for the SF bay area's largest homeless shelter nonprofit:

People who end up homeless long-term usually have negative social behaviors that push others away. When you help them, they don't tell an interesting story, they act angry or yell at you. When you give them money, they don't make you feel you happy, they make you feel afraid or annoyed.

This is unfortunately often due to mental health issues or drug problems. It's very sad, and ends up completely isolating them from all friends, family, and strangers who could help them.

Edit: This article actually puts this into clear terms, long term homeless people are poor "kindees"