From the "Why build Carbon?" section:

> The best way to address these problems is to avoid inheriting the legacy of C or C++ directly, and instead start with solid language foundations like a modern generics system, modular code organization, and consistent, simple syntax.

That last part seems to imply that the authors don't consider C++ syntax to be a good foundation for a modern successor language, so they chose to change it. As to why change it in a Rust-like direction I'd imagine that it's both because it's what fashionable at the time and possibly to attract people who are already familiar with that style.