Because it has always expanded from a single point in space.
Scientists believe that at one point the universe was a single point in space, and has been expanding ever since. Thus the universe, and everything in it, was once extremely organized and tight, but since its contents are moving throughout the universe.
Its entropy, or chaos as I like to call it, was very small since there was once little disorder. But since it is always expanding, spreading its contents over a greater region of space, it's entropy is always increasing.
This is the second law of thermodynamics, that as time goes on the entropy of the universe always increases. Little hard to understand what it is and why it happens, but I hope this gives you greater insight on this topic.