1. It is very fast, noticeably faster than Java, C#, Python, Kotlin etc. Most competitions only have one time limit, and participating in C++ gives you an advantage. Even if competitions have separate time limits for slower languages, it is often less reliable (and from a problemsetter's POV, a major headache).

2. Some other fast languages like C and (or assembly :P) are unsuitable, because programming in them is much less convenient — you lose all of C++-s standard library, which is pretty important.

3. It is the industry standard, so to speak. Almost all CP-specific libraries, tutorials and code examples are written in C++. I can copy-paste someone else's FFT, dynamic CHT, geometry lib or whatever. If I participated in something else, there's a good chance that I'd have to implement a lot myself. Rust or D might be comparable to (or better than) C++ in terms of speed and standard library, but they have nothing like this kind of support.

4. It is supported on every platform.

Currently I see no reason to not choose C++. The only possible reason might be that other languages are more convenient to write in, but the benefits of C++ far outweigh this.