Usually hard to learn and hard to use are very correlated concepts. If something is hard to learn, it usually requires more mental energy to read. For an example closer to APL, mathematical notation is harder to read than the verbose equivalents. You not only need to keep the concepts in mind but also how the symbols are translated to the concepts. Maybe when you’re very familiarized with them they’re as easy and a little bit more terse, but it’s easy to slip back if you’re tired or just woke up less bright that day.

And that’s just in math, where there isn’t as much symbol density as in APL and where symbols don’t depend on context too much (it’s usually not a good practice and avoided if possible to have the same symbol having very different meanings depending on the context).