I think some linear algebra knowledge certainly helps a lot, so that's a good point. A lot of APL doc talks about concepts such as "Rank" and "Inversion" and "Transpose". That is a little math heavy.
The other problem is that I would bet that a lot of the early use for APL was done better by Excel spreadsheets on desktops. By the time APL moved off the mainframe, it was too late. Of course, I'd imagine as far as code maintenance goes, APL beats a very large spreadsheet with a lot of VBA. Excel also has built-in charts and other functionality that is far clunkier, even in the best APLs.
The other problem is that I would bet that a lot of the early use for APL was done better by Excel spreadsheets on desktops. By the time APL moved off the mainframe, it was too late. Of course, I'd imagine as far as code maintenance goes, APL beats a very large spreadsheet with a lot of VBA. Excel also has built-in charts and other functionality that is far clunkier, even in the best APLs.