Command line parameters should be used for things that change frequently, not for configuration variables. You don't want to pass 20 command line parameters every time you invoke a program. You rather want to set your environment variables one time in your shell initialization script (.bashrc, .zshrc, whatever).
You can say there are the configuration files, and sure a complex program should have its configuration file. The problem of configuration files is that they all use a different syntax that you have to learn, you have to write them, back them up, copy around, etc.
Also you don't have a simple way of overriding a configuration file parameter for one invocation of the program, that isn't modifying the configuration file. With environment variables is simple: set them before executing the program.
Last thing is that environment variables aren't specific to a particular program: they are readable by every program executed in that shell. A configuration file cannot be easily shared, since one program can change its format to make it no longer backwards compatible, so you can't rely on reading other program configuration files. While you can with environment variables.
You can say there are the configuration files, and sure a complex program should have its configuration file. The problem of configuration files is that they all use a different syntax that you have to learn, you have to write them, back them up, copy around, etc.
Also you don't have a simple way of overriding a configuration file parameter for one invocation of the program, that isn't modifying the configuration file. With environment variables is simple: set them before executing the program.
Last thing is that environment variables aren't specific to a particular program: they are readable by every program executed in that shell. A configuration file cannot be easily shared, since one program can change its format to make it no longer backwards compatible, so you can't rely on reading other program configuration files. While you can with environment variables.