The advantage of single character concatenated symbols is that it allows your brain to group them rather than forcing you to read them separately. This means you almost unconsciously build up a bunch of set of instantly recognizable "chunks" of symbols that represent concepts you're familiar with. Each of these chunks can represent the equivalent of a huge amount of code in another language.

As the other poster implied, it's the difference between a library of functions and an alphabet for composing functions. Concise symbols allow composition of "words" in a way that longer names really don't. There's a practical language design side too - I don't think the right-to-left evaluation and accompanying concision and logical reading order would work otherwise. Maps in other languages are so clumsy and clunky and backwards compared to the APL equivalent.