Specifically the author's mention of extended character support in Julia for math symbols, as well as the emphasis on matrix support, makes me wonder why APL didn't maintain popularity among the academic crowd.
It doesn't by default have scientific libraries and builtins for solving equations. It has a fast interpreter, but scientific computing often needs much faster.
It was also late getting off the mainframe.
I really like APL, but it doesn't have the horsepower for my needs.