text: There are MANY reasons, I can't really list all of them but here are a few. \n\nFirst off it depends on what type of bridge is needed. Some types can span longer distances but cost more to design and build while others are cheaper but can't reach as far.\n\nAlso, different materials have very different properties. Depending on what is needed Structural Engineers will choosee different materials. Steel for example is very strong in both tension AND compression, however it needs to be kept in good condition weights a lot and also may be hard to build with in some circumstances. Concrete for example has very strong compressive strength and very low tensile strength. This means that while concrete can stand a lot of pressure, if you pull on it, it can crack somewhat easily compared to other materials. Cost of these materials also play a factor.\n\nFor bridges steel and concrete are not the only materials used. I've seen some glass ones, ones made of stone, wood... a lot of different materials. Before you even get into other materials though there are MANY types of concrete and steel to begin with.