Managed strings in allocated memory come with their own baggage. Often allocated memory is not consistent with other important goals or constraints of the program. Programs which use managed string libraries must often deal with classic strings at their boundaries for communication or storage.
Pushing people to managed strings is not a legitimate goal of C library maintenance. It is reasonable to wish for the library maintainers to not view the C library as kindergarten declining useful, simple advances intending to shove people out into another language or library.