The use of C++ enables a couple architectural optimizations that would be difficult or impractical in Java, aside from C++ generally producing significantly faster runtimes. The important difference is that C++ is a systems language -- you could replace C++ with Rust, Zig, etc -- whereas Java is not.

Scylla has a highly optimized low-level I/O path that largely bypasses high-level OS APIs and kernel services that Cassandra and other open source databases tend to use. This will typically generate an integer factor improvement in I/O performance if implemented well and makes a big difference for the kinds of write-heavy workloads Cassandra was built for. It requires taking strict control of low-level memory access and behavior, which (for better and worse) is the default case in C++. This code is intrinsically non-portable.

Additionally, there are some important classes of throughput optimization that are incompatible with garbage collection. In principle you can abuse Java to effect these optimizations but it is much easier to implement these optimizations in languages that don't have a garbage collector. If absolute performance is the objective, like Scylla, it is easier to do the implementation in a language that won't fight your intent every step of the way.

tl;dr: The performance isn't so much that it is written in C++ but that C++ makes critical optimizations relatively straightforward and economic to implement.