On some fundamental level Processing and its ilk succeeded because it just worked and the time from opening the main web page to typing some code and seeing a graphical, animated result was as close to zero as anything could be, which is nearly true today even. It's impressive. The documentation was exceptionally good: every method documented with multiple graphical examples, almost exhaustively. I can't think of anything as accessible from the same era.
From previous eras, maybe Flash, Visual Basic, QBasic and friends -- but they were nothing like Processing.
No real need for libraries, a huge batteries-included standard library. You can practically do rudimentary computer vision with a small subset of the builtins, all of which fit on a single monitor. It's pretty nuts.