Why hydrogen is not used to compare the relative atomic mass

[Johannah Wu]

Is that because of some historical reasons or E=mc2(i mean something about binding energy)?

 

[jtbell]

See the section "History of the atomic mass unit" on this page:

http://www.sizes.com/units/atomic_mass_unit.htm

(I first went to the Wikipedia page about the atomic mass unit, which doesn't say much about the early history; then followed the "external link" at the end of the article.)