Well, the easy answer is that some materials are just better suited to one purpose than another. The material you use for a particular job really depends on what you're trying to do.
Materials engineering is all about knowing what materials to use for a job.
It's also about studying the properties of materials, developing new materials and finding new ways to work with existing materials. Engineers who specialise in materials work in many different fields. Some work in the petrochemical industry because many modern materials, such as synthetic rubbers and plastics, are made from the by-products of petroleum refining. Others work in the automotive and aerospace industries. Engineers in these fields are really interested in harnessing the heat resistant properties of ceramics for use in engines.