This was the era before optimizing compilers.[1] The overwhelming majority of commercial games were shipping hand-coded assembly still. Forth had the advantage of low overhead, no-worse-than-a-compiler speed, and better-than-assembly productivity. It was a small time window, but a good fit in the moment.

[1] Non-trivial optimizations were just starting to show up on big systems, but Microsoft C in 1985 was still a direct translator.