Because pretty much any language that's been touted as a replacement has problems which need solved before they'd really be viable. They're either single vendor with no independent standardization, like Rust or D, or have lower performance like go, or both. Once both problems are fixed, it'll be a lot easier to start looking at alternatives. Until then, there's not a language that settles into a lot of the niches C and C++ operate in.

This case specifically, a lot of the issues that allowed this to happen were logic errors which would have been valid in most languages. A functional language would not have helped very much here.

C and C++ are still used because they work, and have huge ecosystems around them. There's a lot of tooling around them, the compilers build fast, code, and are available for pretty much every planet under the sun. This particular case, it would have helped a lot if the developers looked at the warnings of the questionable code they had made, but they seem to be of the "it compiles, ship it" type that would have let those things fly regardless of the language.