Consider the formula $$\text{Speed} = \frac{\text{Distance Travelled}}{\text{Time Taken}}$$. If we want to measure the instantaneous speed, we shouldn't wait too long to measure it. In other words, we should choose as low a value for Time as possible.
Of course, we can't choose the value of Time to be $$0$$! Because we cannot divide any number by $$0$$. But, the closer the value of $$\text{Time}$$ to $$0$$, the more instantaneous the value of speed.