I know especially as an old Lisper, however, J's succinctness allows you to fit a lot on a one-line REPL idea. No blocks, curly braces, or indents like when using other languages' REPLs. Also a simple backspace, and you've deleted a noun, and one character or two, and you're creating a lot more than full words in other PLs. Tools for Thought by Ken Iverson, like using math symbols, but on a REPL.