[hinkley]

Because in order to measure something in terms of a constant, you cancel out all the other units except the one you're measuring.

I.e., if you know how to measure a second, you could define the meter based on the speed of light by creating light in a vacuum and timing it.

Meanwhile, you can't define a meter or a second in terms of pi, because it has no units. It's just a ratio of the surface area of certain classes of object or shape to thheir dimensions.