Loosely stated, imagine that atoms can be arranged in a very large number of configurations. Only one of those will be "perfect", i.e. a perfect crystal structure. The rest will be defected in some way or other. As the size of the system increases, the amount of defected configurations also increases and the greater the chance it will have a defect. This is due to entropy/thermodynamics, depending on your interpretation.

That's it in a nutshell, I'm sure someone else can clarify.