Productivity, and the general labour intensity.
Formal verification is not critical for the lion share of all software, and people obviously would like to save man hours by not dealing with it.
Second is the availability of developers. Not many people even now what formal verification actually is, let alone see rationale for learning verification driven development.
ADA is a very productive language for embedded systems, much more so than C. But people are afraid of the language, because they think it's difficult. People don't want to learn it because they think it's old, although ADA 2012 is a much more modern language than C11, it hasn't stagnated as much, but it also evolved more deliberately than C++, keeping it's focus on useful features, instead of adding all the features. At the same time it makes a lot of things easier in an embedded context, because it was designed exactly for this use-case.
The only issues are the lack of developers and the ugly syntax, but I think any C developer could learn what they need to be productive in a month, and the syntax you can get used to.
Not only that, but you can also opt out at compile-time if you need, for performance reasons.
You also have (optional) runtime bound-checking for scalar values (aka "integers"), and you can even specify the bounds yourself (like "A number between 3 and 17").
That said, Delphi is blazing fast so Ada will seem a bit slower, typically… but [IME] it's generally not going to be something as slow as a lot of C++ programs.
The parts of the syntax I'm very familiar with just get treated by my brain just the same. It's not "busier" in my head because words are written out. But things I'm not so familiar with I can basically just read. And anything really unfamiliar with is easy enough to search.
Syntax stuff falls out of my head pretty quickly if I'm not using it, so I find this particularly useful.