Presumably they missed it the same way the developers of those other things you listed did. I can only assume they didn't know about these other problems when developing their YAML parsers.
Of course, you could argue that developers should always be thinking about and searching for security related issues in whatever field they're working in, but that doesn't appear to be the norm at the moment.
Of course, you could argue that developers should always be thinking about and searching for security related issues in whatever field they're working in, but that doesn't appear to be the norm at the moment.