The current Universe has a horizon distance of
horizon ~ age x speed of light ===> the older the Universe is, the larger the horizon and therefore the larger the size of the region which can communicate.
A simple calculation indicates that at the time of recombination, material which was in causal contact (able to communicate) are currently separated by around 0.8 angular degrees on the sky. this means that the sky could only mix over separations of 0.8 degrees and so, there is no rational reason to believe that the sky should look the same everywhere! This is the Horizon Problem.