Why is Carbon 12 isotope used as a standard reference for measuring atomic mass?

Earlier scientist used hydrogen as refernce, but by doing so atomic mass od most eemnts came out to be fractional. So the refrence was changed to oxygen-16. But the ecientist picked up natural occuring oxygen which is a mixture of O-16, O-17 and O-18. This led to confusion.
So they changed the refrence to Carbon-12 .
The adavantage of using this refernce was that by taking it as refernce the atomic mass of all other elements were found exactly.
Also, Carbon-12’s atomic mass could be measured particularly accurately compared to the other elements on the periodic table.