As with all things government procurement, it solves yesterday’s problems tomorrow.
That mandate made sense with the 70s-80s model of big iron systems built in mainframes/vax/etc or other low change rate systems. It lowered the risk that the customer would lose control of the applications that it built.
That was and is a real problem. I’ve worked in various .gov roles outside of federal, and many key business systems have black boxes that nobody can edit or sole-source proprietary software from some long defunct company.
https://news.ycombinator.com/item?id=7824570
That's a submission I made now 7 years ago that has both an article on the topic and a lot of good discussion.
Summary: Complex language at the time impeded compiler development. Compilers were not free (like C with Unix systems). People don't like being told what to do (Ada was initially mandated by DoD for new systems). The compilers remained very expensive into the 90s, by which time open source compilers were out there but the momentum was against Ada. Even after the open source compilers, commercial ones are still (today) very expensive.