Why is nickel more abundant in space?

As planets form, the materials of which they're composed differentiate. That largely means that the denser ones settle to the core. The Earth did not completely differentiate, which is why we can find some heavy things like gold and iron near the surface; however, most of the Earth's metals, including nickel and iron, are in the core, beyond our reach. Asteroids, being much smaller than planets, are hardly differentiated at all, which means their heavy (dense) components are as accessible as their light ones.