And they no doubt picked a pre-ROHS processor because it A: Does what they need it to do B: is more reliable than a more modern design in that environment.

Enhanced reliability from:
1. Solder with lead in it to prevent tin whispering
2. Larger feature size on the ICs to increase MTBF from radiation (cosmic ray) damage in space.

It was probably a rare project where board real estate and BOM cost truly don't matter. I'd go with a wider pitch part too. I'm unclear if tin whiskering is worse with Pb-free solder. Maybe something else on the board didn't like a 240C process, which is fine as long as there's not a RoHS BGA on the board. I always rework and prototype with Tin Lead solder even if the board will have to be RoHS. RoHS was less common yet when they did the first spin, probably years before launch.