Because a dev's IDE project file can differ between every other dev, so committing it doesn't make sense. On the other hand, adding it to the .gitignore solves the problem of having differing config files.
But why is a “config file” (which is usually no more than a few dozen bytes) a problem? If you agree to add the name of the file in your .gitignore, why do you even bother to ignore it in the first place. It pollutes the .gitignore file as much as it pollutes the root directory…