Because they're benchmarking the implementation of async IO, and showing the results against libraries that folks use. This is not a common pattern in C/C++, so the libraries there are fewer and less people have used them.
But, if you want C++, check the data from the original benchmark which was linked: https://www.techempower.com/benchmarks/#section=data-r12&hw=... . The Java one is slightly faster than the C++ one (more like "just as fast", since the difference is tiny). And there are 7 more java libs (half of them with recognizable names) before the next one (which I've never heard of), which underlines my point about this being more common in Java than C/++.
The benchmark wasn't shown to prove that Rust is the fastest language in the universe. It was shown to prove that the Rust futures implementation is competitive with the others in use today, ones which people would recognize.
But, if you want C++, check the data from the original benchmark which was linked: https://www.techempower.com/benchmarks/#section=data-r12&hw=... . The Java one is slightly faster than the C++ one (more like "just as fast", since the difference is tiny). And there are 7 more java libs (half of them with recognizable names) before the next one (which I've never heard of), which underlines my point about this being more common in Java than C/++.
The benchmark wasn't shown to prove that Rust is the fastest language in the universe. It was shown to prove that the Rust futures implementation is competitive with the others in use today, ones which people would recognize.