Question

why is carbon atom used as reference point for relative atomic mass of other atoms

Answer 1

One scale was used by physicists; the other by chemists.  Both were based on weights compared to Oxygen, rather than Hydrogen.  Oxygen was used because it combines with a lot of things to form oxides.  This made it a better choice as a standard because of the ease of chemical analysis.  Oxygen was set to have an atomic mass of 16, which was just about 16 times as heavy as Hydrogen being 1.   Unfortunately, Chemists picked naturally occurring Oxygen, which is a mixture of isotopes of Oxygen-16, Oxygen-17, and Oxygen-18.  After all when you made an oxide of an element you would do so in naturally occurring oxygen.  Physicists picked the pure isotope Oxygen-16, because they tended to make their measurements on the basis of mass spectrometry.

Though the ratio of any two atom’s masses was the same on either scale, it was horribly confusing, so in 1961, a compromise was reached. Instead of using either Hydrogen, or Oxygen as the standard,

the isotope of Carbon with 6 protons and 6 neutrons in its nucleus (Carbon-12) was given a mass of exactly 12. It was a good choice, since it was in between the two previously used standards, and meant that nothing had to change too much. Plus, Carbon-12’s atomic mass could be measured particularly accurately compared to the other elements on the periodic table.