[ndriscoll]

Because copyright doesn't expire on human timescales and it's legal to use cryptographic methods to prevent compatible hardware/software, so things wouldn't be on an even playing field.

If you had to submit source code to the copyright office to be granted a copyright, and it expired after ~10 years (at which point the source is published), and anticompetitive, anticonsumer hardware locking methods were illegal, you'd be looking at a reasonable trade again, and copyleft would be essentially redundant.