They used two digits for the year because they didn't think their code would still be in use that far in the future. They assumed that when someone 40 years in the future wanted to program a computer they'd write something for it.
They probably thought the same thing about language. At the time, they were writing for their specific piece of hardware. They assumed that when someone wanted to run their own new computer they'd write their own language and operating system to do it, just like everyone else always had.