Thermal conductivity is important. Copper would be one of the "best" choices when it comes to common metals with high thermal conductivity, but it is very soft and is prone to oxidation as a bare metal. Therefore brass is used; it is alloy an of copper and zinc that is known for it's anti-corrosion properties, low friction properties, and strength. The brass used probably is high brass with a copper content of about 65%. Higher copper content is good since copper has roughly 4 times better thermal conductivity than zinc, but the zinc helps keep the hot end strong, clean, and free flowing.