There's a big difference when choosing technologies for a home computer versus a safety-critical system that will run for decades. Seems reasonable they would've chosen a proven, if slightly outdated, tech over a newer one that had less data to back up its long-term reliability.

And of course we can get into the discussion of cost: floppies are way cheaper than hard drives, especially for the presumably-small amounts of data that are needed for such a control system. A 500MB hard drive was probably overkill.

I am of course speculating, but I don't know why we should assume that the engineers working on the project originally made a silly decision. They almost certainly weighed the available options to deliver the project within the defined constraints of the system.

Many computers in the late 1980's came with hard disks, but the cheapest ones did not.

Yeah, on further reflection, I recall my parents having a 286 with a hard drive. Still don't get why they couldn't find a 5.25" floppy. Then again both types of floppy drives are still quite obsolete....