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It apparently was chosen because it's the starting sound of "turn": Hartl chose tau to represent 2pi because it nicely ties in with the Greek word “tornos,” meaning “turn,” and “looks like a pi with one leg instead of two.”

https://blogs.scientificamerican.com/observations/the-tao-of...

There was an earlier effort that used a new "two pi" symbol consisting of a "π" with an extra leg in the middle: https://www.math.utah.edu/~palais/pi.pdf.

Funny coincidence: π with an extra leg is the Cyrillic cursive letter for the sound t.

https://tauday.com/ is a good entrance to this particular rabbit-hole.

Doesn’t Tau (the letter) look like half of Pi? Isn’t this a lost cause already?

I think it's a won cause already.

Look up the Tau Manifesto: it’s all explained there.