Umm... i think its because the longer the string the further distance the pendulum is able to swing, to the time taken will be longer than a pendulum with a shorter lengthed string.
The length of the string determines how much the pendulum is able to swing, so a short lengthed pendulum would take a short duration of time for one oscillation because it cant 'move' / 'swing' as much.
Am i making sense? Im not good at explaining things but yh. Hope i helped a tiny bit atleast. trying drawing a diagram of a pendulum with a long and short string, may be that would help my explanation.