Why does the phase diagram of water have a negative slope?
Unlike most materials, water is more dense as a liquid than as a solid. So increasing pressure favors the liquid instead of the solid.
Because water tends to form hydrogen bonds, its (ordinary) solid phase does not have a close-packed structure. The molecules in water ice take up extra space in order to make their hydrogen bonds line up and make the most stable solid at ambient pressure:
When the ice melts, this rigid hydrogen-bonded structure collapses and that makes the liquid more dense than the solid. So compared with most materials going to higher pressure favors the liquid instead of the solid and the solid/liquid boundary on the phase diagram slopes the "wrong" way.