The core language of J was an improvement on the APL back then, but modern APLs have since borrowed many features from J. Iverson's choice of ASCII was because

The core language of J was an improvement on the APL back then, but modern APLs have since borrowed many features from J. Iverson's choice of ASCII was because he wanted to make it more accessible to schools that couldn't get the equipment necessary to display APL glyphs. Note that J was released the year before Unicode.