Why is Earth's outer-core liquid?
First, you need a phase diagram that goes to higher pressure. The pressure at the inner/outer core boundary is over 300 GPa. The one in the question would only get us into the mantle:



(link to source)

A typical temperature and pressure at the outermost part of the core would be 3750K and 135GPa, which is in the liquid region of the phase diagram.  

For more data on pressure and temperature as a function of depth see this University of Arizona source.  All appropriate credit to Marcus Origlieri.