The reason you are asked not to use all caps, is that it's considered to be "yelling". It's also easier to read a question or answer when it's written in the conventional way. And, writing in all caps is considered by many to be bad manners.
Because the ask.com system can't read the comment, question, or answer when it's written in all caps. I saw Lunger say that one time. And plus, it looks like you're shouting which can come off as rude or mean to some people.