Why is the wire in a heater made of nichrome and not copper?

Heating elements need to convert electrical energy into a lot of heat, and the heat produced depends on resistance through H = I²Rt. Nichrome has a high resistivity, so a short, manageable coil of it offers enough resistance to get red hot and give off heat. Copper has very low resistivity, so a copper coil would barely heat up; it is good for carrying current to appliances without wasting energy, which is why connecting wires are copper. Nichrome also has a very high melting point, so it does not melt at the high temperatures a heater reaches, and it does not oxidise or burn away easily in air even when glowing. These properties together make nichrome ideal for the heating element while copper stays in the supply wires.