Linked from
The 17 pages that link to Carbon-12, each with the reason it gives.
IsotopeBroader topic: It is the reference isotope that defines the atomic mass unit.
Atomic massBroader topic: Its mass established the modern atomic-mass reference.
CarbonBroader topic: Its mass defines the atomic mass unit used to compare atomic masses.
Carbon isotopesBroader topic: Its especially stable nucleus makes it the reference for atomic mass.