The overhead that is assumed unimportant by most functional languages (copying is cheap, GC is fine, etc.) is intolerable in key situations and problem domains. In C++ all overhead must be optional.
That being said, nearly all C++ applications can benefit from immutable classes, algorithms that take closures, and so on. But C++, like C before it, is really just an abstraction of the hardware and OS below it. That hardware mutates. That pool of memory changes size and locality. The closer your problem domain is to the metal (especially drivers and highly-optimized code), the more you appreciate mutable state, at least given the current state of computing.
Dealing with complex immutable data structures without GC is either extremely hard or inefficient (and usually both). C++ standard library does not offer any helping hand here either. Additionally C++ logical memory model (flat, uniform memory) does not describe the hardware memory model well - it is already an abstraction. An memory access isn't uniform and mutations can cost you really very much. In some cases immutable structures + little copying is often the way to go, especially when we aim at parallelization.
That being said, nearly all C++ applications can benefit from immutable classes, algorithms that take closures, and so on. But C++, like C before it, is really just an abstraction of the hardware and OS below it. That hardware mutates. That pool of memory changes size and locality. The closer your problem domain is to the metal (especially drivers and highly-optimized code), the more you appreciate mutable state, at least given the current state of computing.