I would guess that two big issues for that are lack of function overloading and that macros are pretty good.
A lot of the patterns of working with variadics in c++ take advantage of function overloading. Without function overloading, those patterns can become harder.
Secondly, Rust macro system is crazily powerful especially procedural macros. That power disincentivizes developing other approaches to meta programming. In C++ macros are clunky and dangerous, and that has incentivized developing more type based meta programming techniques to get away from C macros.
One problem with the macros approach is that you get really hard to understand error messages. For example, if a user wrote a method where one of the arguments did not implement ToPolarValue, they would get the following error:
fn blah(blah: Blah)
^^^^^^^^^^^^^^^^^^^ the trait bound Method<_, _> is not satisfied
As opposed to something like this with variadic generics:
fn blah(blah: Blah)
^^^^ the trait bound ToPolarValue is not satisfied
proc macro diagnostics are a cool feature, but they would not help here. Here, macros are being used to generate implementations of a trait. If the trait bound is not satisfied, you have no way of knowing why other than that there was no implementation generated for it, hence the confusing error message. With variadic generics however, you could write something like:
Fn(..Args) -> T
which I assume would let the compiler know why the trait bound is not satisfied and output a nicer error message.
I would guess that two big issues for that are lack of function overloading and that macros are pretty good.
A lot of the patterns of working with variadics in c++ take advantage of function overloading. Without function overloading, those patterns can become harder.
Secondly, Rust macro system is crazily powerful especially procedural macros. That power disincentivizes developing other approaches to meta programming. In C++ macros are clunky and dangerous, and that has incentivized developing more type based meta programming techniques to get away from C macros.