Why is the core of Earth in a reduced state? ($\text{Fe}$ and $\text{Ni}$)
Because there wasn't enough oxygen to oxidise all of it. There is only so much oxygen on Earth. Most of it went to oxidise the elements that have higher affinities to oxygen: silicon, magnesium, calcium, aluminium, etc. Iron and nickel have lesser affinity to oxygen, so you start by oxidising some of it, until there is no oxygen left.

But there is still unoxidised iron and nickel left! So this just sinks to the core.

Further reading.