The concept of pi had been around for several thousand years before it was properly named and given a symbol. Mathematicians were content with referring to it as “the quantity which when the diameter is multiplied by it, yields the circumference” until William Jones was probably tired of reciting that novel of a name every time he wanted to talk about pi in his math books.
He didn’t simply rummage through the Greek alphabet, searching for the coolest looking symbol he knew we’d revere for years to come. He selected the name with a purpose in mind. The Greek word for “perimeter” is “perimetros”, which starts with the 16th letter, π. Coincidentally, the Latin equivalent in the English alphabet is also the 16th letter.