I remember taking Computer Science courses in the late 90s. The professor of CS explained to our meager group of CS majors that he was going to teach us C, rather than COBOL or Ada, because it would be more useful in general. At the time, they were the major programming languages colleges were teaching back in the day (every major university in the area with a CS degree taught one of those two). Why would they teach older languages? Because you could still get a job writing code in them. This was around the time the ISS was being built.
The reason Ada came about was because they needed mission critical software that was reliable and Ada was literally made to fit the bill. NASA still has one of the costliest software errors in history (back in the dark ages of punch cards)
A bug introduced by a punch card turned out to be one of the most expensive software bugs in history, when it led to the destruction of the Mariner 1 spacecraft in 1962 (cost in 1962 dollars: 18.5 million; cost in today's dollars: $135 million), before it could complete its mission of flying by Venus.
However, there was a fatal flaw in the software of that guidance system: when the equations that would be used to process and translate tracking data into flight instructions were encoded onto punch cards, one critical symbol was left out: an overbar or overline, often confused in ensuing years with a hyphen. The lack of that overbar, essentially, caused the guidance computer to incorrectly compensate for some otherwise normal movement in the spacecraft.
Ada's main feature is stability
With its emphasis on sound software engineering principles Ada supports the development of high-integrity applications, including those that need to be certified against safety standards such as DO-178B and security standards such as the Common Criteria. For example, strong typing means that data intended for one purpose will not be accessed via inappropriate operations; errors such as treating pointers as integers (or vice versa) are prevented. And Ada’s array bounds checking prevents buffer overrun vulnerabilities that are common in C and C++.
NASA tried to create it's own Ada-esque language called (I am not making this up) HAL/S which the Space Shuttle used and... not much else. All the ground equipment would wind up using Ada by then (standards!)
A proposal for a NASA standard ground-based version of HAL named HAL/G for "ground" was proposed, but the coming emergence of the soon to be named Ada programming language contributed to Intermetrics' lack of interest in continuing this work.
Oh, HAL/S has one other minor problem
The weakness of Hal/S is that it is readily available only when hosted on the IBM 360/370 series of mainframe computers. Intermetrics has also hosted Hal/S on the Modcomp IV and Classic, and on Data General Eclipse minicomputers. But the Modcomp implementation is used only by JPL, and the Eclipse development is no longer in use because address-space limitations reduce compile speed to around 30 lines/minute.
As such, it's not surprising that in the early 90s (when PCs were on the rise) it was much cheaper to source more standard computing devices (my bet is IBM was able to make Ada run on more standard hardware, since they were already supplying astronauts with Thinkpads round that time). Fun fact: Thinkpads running Linux are used to interface with the station computer
"It was just the applicational needs," he said, describing how the laptop software that talked to the station's command and control systems in those early days was designed to run on a Linux-based OS.
By contrast, the curiousity rover (early 2000s) ran on C
It's running 2.5 million lines of C on a RAD750 processor manufactured by BAE.