[upofadown]

>... but why is it preferred over portable C,...

C normally involves compiling the entire program at once. Even if you use a interpreted C of some sort, C is a lot more verbose and involves more typing.

This is important when testing and evaluating new hardware. All you need is a serial port or equivalent connection to a forth to allow you to exercise the hardware in a very interactive way. In some cases you might end up writing the final program by accident.