This question can be explained in the light of following concepts. In case of alloy the metals are mixed by melting them. The main point to note is that the proportion is not fixed in case of alloys. But in pure substances the proportion is fixed . And they make same kind of particles .
Here are the characteristics of pure substances They are composed of same kind of particles. Can not be solved into its components by physical method. Have characteristic melting and boiling points. All the particles or the bulk show similar chemical and physical behavior.