Here's why:
* Carbon-12 is the standard: Carbon-12 is defined as having an atomic mass of exactly 12 atomic mass units (amu). This means that one amu is equal to 1/12 the mass of a carbon-12 atom.
* Relative comparisons: The atomic masses of all other elements are determined relative to carbon-12. For example, if an atom of element X is twice as heavy as a carbon-12 atom, its atomic mass would be 24 amu.
While sulfur-32 was used as a standard in the past, it's no longer the case. Carbon-12 became the accepted standard for atomic mass in the 1960s.
