Both SPARK and Rust provide memory safety, so it's perfectly reasonable to use popularity as a "tie breaker". Applications are written by people; the more people who can usefully contribute, the more work can be done.
I think this "popularity" reasoning is flawed. It completely misses the context of the tool. Just ask yourself -- are there many UNIX CLI tools written in Ada/SPARK? I'm sure there are some, but not aware of any I use regularly.
Meanwhile, there is a Cambrian explosion of Rust CLI tools taking place.
"Popularity" is really a surrogate for "popular, accessible, infrastructure for an OSS CLI tool for UNIX", a place where Ada AFAIK has not been a first choice. If we were building firmware for a missile guidance computer, maybe the reasoning would be different?
But it's still not a sufficient reason, so I'll add another: People really like using Rust to make these types of tools. It's amazing to me that people are so willing to discount as non-technical, and therefore unworthy of discussion, lots of people really like using Rust.