Why is Earth's Core Iron? The two major constituents of the Earth's core are iron and nickel.  In documentaries and scientific conversations, iron gets more attention than nickel, probably because it makes up a bigger percentage of the core.

But before I consider this scenario—where it was nickel that makes up the bigger percentage instead of iron—I must ask:

Why does iron make up the majority of Earth's core as opposed to nickel?  Does either metal have an advantage over the other?  If so, which advantage?  If nickel is the number-one metal of the core instead of iron, how would the magnetic field be affected?
Simply put, the amount of iron in the solar system/universe is higher than nickel due to a process called nucleosynthesis. 

Basically at it's origin the universe comprised only of hydrogen, through nuclear fusion heavier elements were/are being created. Fundamentally, hydrogen is fused into heavier elements in the star, our sun is currently turning massive amounts of hydrogen into helium through the proton-proton chain.

Eventually as a star gets older you start seeing fusion of heavier elements, helium, carbon, nitrogen, etc up until you hit iron at which point you hit what is known as the iron peak  where fusing iron atoms together requires more energy than is release. Heavier elements are all formed during supernovas and thus we see lower level of abundance. That being said the abundance of nickel is still higher than all the other heavier metals.  

https://en.wikipedia.org/wiki/Stellar_nucleosynthesis

http://physics.info/nucleosynthesis/