Rust's design has given more thought for how to work in the kind of programming environment that the Linux kernel sits in than C++ (even C++11) has.
Specifically (and this is no longer quite eli5), Rust's freestanding mode--working without a standard library--is somewhat fuller and more fleshed out than the C++ freestanding mode.
Now, the sibling comment that points out that C++ gained a bad reputation pre-C++11 is probably a bigger reason for it not being seriously considered whereas Rust is, but there are some technical reasons where Rust does better than C++.
Specifically (and this is no longer quite eli5), Rust's freestanding mode--working without a standard library--is somewhat fuller and more fleshed out than the C++ freestanding mode.
Now, the sibling comment that points out that C++ gained a bad reputation pre-C++11 is probably a bigger reason for it not being seriously considered whereas Rust is, but there are some technical reasons where Rust does better than C++.