They fear mistakes, believe things that were before possible, now impossible and are educated to think differently than they would have as kids. Think about when you learn another language now, you may get words wrong then you won't try. Whereas kids try and try until they get it right, until they start to fear mistakes, at which point they stop learning as fast and being creative.
Ken Robinson captures this concept really well in this 2006 Ted.com speech titled, 'Ken Robinson says schools kill creativity'.
http://www.ted.com/talks/ken_robinson_says_schools_kill_creativity.html