Answer:
Because time is independent of distance, and distance is dependent of time.
Explanation:
Usually, on any graph, the independent variable is plotted on the x-axis and the dependent variable is plotted on the y-axis. Because of this, time, which is independent (time happens regardless of any other factor), is on the x-axis while distance, which is the dependent variable (can only take place in time), is on the y-axis.