In 1961, the reference for the atomic mass unit (amu or u) was changed from naturally occurring oxygen with a value of 16.000 u to one based on a single isotope of carbon (carbon-12). why was it changed

To determine the mass of an atom in terms of atomic mass unit (a.m.u.) the standard should have the fixed mass otherwise the calculated mass of the other atoms will get error. Initially oxygen was used as the reference standard for the calculation of other atoms mass. But after the invention of different isotopes of oxygen i.e. and which ratios with respect to is 1.002 the mass of standard oxygen became unstable. Thus the mass of the new atom have error. Thus in 1961 the standard reference is canged to carbon-12. After that onward the mass of the other atoms are measured with respect to carbon.