Because OpenFirmware, the more or less standard bootloader of PowerPC and Sparc specified forth as it's command language. Even let you write PCI option ROMs in forth to have ISA independent boot drivers.
Open Firmware written by Mitch Bradley is actually just a byte-code Forth custom extended to be the system firmware.
:)
So it's not that the language was specified but rather the language is the Firmware. By using byte-codes for the Forth keywords and allowing new wordlists to be created and extended you get a nice virtual machine that has the low-level chops to let you write device drivers and such, as you mentioned, while keeping the object code very small.
It quite clever. Unfortunately Forth itself is so "other" that it can never win the popularity contest. However like Lisp it is one of those mind expanding paradigms that should be sampled for the insights into minimalist computing that it provides.
