Why the Earth inner core is solid and the outer is liquid?

The inner core, is solid and about 1250 km thick, the outer core (about 2,200 km thick) is so hot that the metal is always molten, but the inner core pressures are so great that it cannot melt, even though temperatures there reach 3700ºC.

In general, the solid state in almost all materials, is denser than the liquid, because it is better packed, and has less random thermal motion in it. So that's way the inner core is solid and the outer is liquid. The high pressure in the "middle earth point" are so great that the freezing point is lower, even at this colossal temperatures of 3700º C, where at normal pressure it would still be liquid.

Image taken from: www.universetoday.com