This isn't just a C++-ism. The null pointer constants are only more prominent in C++ because of the rejection of void * even though it isn't any "safer" to have a special integer literal with the same semantics. Ultimately, it all comes from K&R C.
There are legitimate cases where you need a function pointer assigned to address zero (reset vectors commonly). The correct behavior in C is ambiguous if the null address isn't also 0 since the standard doesn't call out special behavior for function pointers. That wouldn't be the case if nullptr had been standardized earlier and there was no need for the magic 0 as a null pointer constant.
There are legitimate cases where you need a function pointer assigned to address zero (reset vectors commonly). The correct behavior in C is ambiguous if the null address isn't also 0 since the standard doesn't call out special behavior for function pointers. That wouldn't be the case if nullptr had been standardized earlier and there was no need for the magic 0 as a null pointer constant.