GCC is a great C++ compiler that works on more platforms than the rust compiler. Clang, another great C++ compiler, works on at least as many platforms as the rust compiler.
Again, why would this be a reason to prefer rust over c++?
On the off chance that you aren't being deliberately obtuse, here's what's going on:
If, given two situations
1. The the set containing compilers for language L is size 1, and that compiler works on multiple platforms; or
2. The set of compilers is > 1, but there exists at least one compiler in the set which is a) popular for use with language L b) not fully interoperable with the other compilers from the set that satisfy (a); then
situation 1 is the preferable one
Or to put it another way, nobody complains about code not working with compilers that don't exist.
3. The set of compilers is > 1, and there exist at least a popular compiler that works on multiple platform.
Rust is in 1 of course. C++ is both in 2 and 3. In particular for C++, the compiler in 3 works on a strict superset of all the platforms supported by rust.
As you are free to support only this C++ compiler and ignore every other C++ compiler (you must not care about multiple implementations otherwise you wouldn't think that 1 is a good solution), it seems to me that 3 is strictly better than 1.
edit: I do concede that the existence of a popular but non-interoperable compiler[1] does fragment the community and it has been an issue in C++. Things are getting better though, and if rust becomes popular enough it might become an issue there as well.
Again, why would this be a reason to prefer rust over c++?