Why does disorder (entropy) increase in the same direction of time. as that in which the universe expands?
Why can't entropy decrease in the same direction in which it increases ?
Entropy is the only quantity in the physical sciences that requires a particular direction for time, sometimes called an arrow of time. As one goes "forward" in time, the second law of thermodynamics says, the entropy of an isolated system can increase, but not decrease, Universe can seen as one big isolated system in which total energy is always constant. Hence, from one perspective, entropy measurement is a way of distinguishing the past from the future. https://en.wikipedia.org/wiki/Entropy_(arrow_of_time) - wiki for relation between time and entropy.
https://www.youtube.com/watch?v=870y6GUKbwc - Basics of entropy.
Highest state of entropy is achieved when the system is in equilibrium, to have this equilibrium, energy entities (mass and other energy radiating sources) should be enough apart from each other that no more energy interaction(give and take) could happen and energy is distributed evenly in the universe, thus expansion.