Because it's not the prototype of TIE fighters.
It's the prototype of the TIE Advanced.
But it's not the only prototype. The TIE Advanced x1 was the first prototype of the eventual TIE Avenger.
Why muddy the waters by throwing the word Prototype and abbreviating Advanced when you could either maintain the usual prototype naming scheme with the x1, x2, v1 (instead of making it yet another title card) or just renaming the ship type entirely?
