Lack of education? Yesterday there was a blog post lamenting computer illiteracy in which the author asked how many people could tell the difference between the Internet, world wide web, a browser and a search engine without referencing Wikipedia?
Well, in America we have widespread political and legal illiteracy as well. I'm not talking about citizens not reading the Constitution, I'm talking about most people being unable to paraphrase the Fifth Amendment when asked. Aside from "plead the fifth" most people in America probably couldn't tell you what the amendment means. Yes, it applies to more than just courts. You won't be held to incriminate yourself, your property can't be unjustly seized without compensation, you must have due process and a jury, etc. etc. etc.
Americans lack basic Bill of Rights knowledge, because honestly, they don't teach it enough in schools. It's great that we have to stand up and pledge allegiance, but it would be nice if we had to repeat our own rights. As an American it bothers me.
Most people don't exercise that right when a police officer (or any other agent) speaks to them because it seems innocuous, and they don't think that their rights apply to a simple conversation with law enforcement.
All that said...I wouldn't say it's always unintelligent to speak to law enforcement without a lawyer...but still, most people should be aware of the right, and take judicious advantage of it.