Why do some people make fun of the mentally handicapped or autistic people if you will?

I was on the bus this morning an there was this one guy I'd say he was in his 20's maybe anyway you could tell he was slow and everytime he would tell people that they could sit next to him on the bus or compliment people on their shoes or whatever these I guess teens would laugh at him and make some rude remarks which sort of irked me after awhile and lead me to tell them to knock it off. So why are some people so cruel?