I think it's more of a progression thing. The config file starts out pretty simple, enough where one of the text file formats is clearly the right solution and code is over-complex. But then it grows and grows, continuing to accumulate more depth and use more features of the format, and maybe some hacky extensions. At some point, it gets complex enough that, if you designed it from the start with that feature set, being code would obviously be the right solution. But now the transition over is hard to pull off, so it tends to get put off for longer than it should be.