Because it is the ratio between the diameter and circumference. Hope it helps and pls mark as the best answer. :-)
The Brainliest Answer!

The Story of Pi

Now some other crazy nerds like Archimedis. What they did is , measured the circuference of the various size (big and small ) circular objects using thread . They the measured the diameter with a scale . Divided both and found it always gives a constant value = pi ,no matter how big or small circle they chose .

Defination :

Thus the defination : Pi is the ratio of circumference to its diameter. i.e. Circumference / 2r = Pi => Circumference = 2r(Pi) .

Fun Fact:
Now you know that Pi was derived from Circumference and not the Vice Versa.
1 5 1