APL predates ASCII by a couple years.
It originally wasn't even intended as a software language, but rather a uniform mathematical notation in the style of curry's combinators, but more practical for describing non trivial algorithms.
So he was in an era where the expectation was if you were typesetting a mathematical monograph you'd already be doing stuff like swapping the balls on your IBM typewriter for math symbols.
It's not a choice you'd make today obviously, but it was entirely reasonable then.
As for why it persists, simple answer is APL fans like it that way. It's trivial to translate to some ascii text representation. I think anyone strongly motivated to do that just switched to j, k, or if even those are two weird goes to numpy or such.