Because it's easier to multiply than to divide. You could also determine your TOD by dividing your FL by 3 i.e. FL330/3=110nm. The difference with the multiplication method is the deceleration segment.
I started a thread not so long ago asking how many pilots on the 737 multiply the altitude to lose to FL100 by 2.5 rather than 3, to account for the steeper descent gradient at high altitude and high speed. The answer to that was, nobody.
Keep. It. Simple.