It isn't black and white. The OGL has many advantages:
It is considerably easier to use for second and later generation works, has a built in mechanism for trademarks, plotline, and character protection, and (most important) it has a built in existing commons and it is gigantic.
CC isn't as useful for RPGs because it was created for mostly 'serious' work -more academic, less literary-, and to prevent plagiarism, not to enable it. CC doesn't asume your work will be remixed and modified in a seamless way, but rather quoted faithfully piecemeal, and it doesn't give a lot of guidance for doing the opposite. Particularly the requirement to indicate clearly how you are modifying the work, which is rather unwieldy and leads to disjointed and unusable rulebooks. And no, the "reasonable" doesn't help, because it is a very subjective qualifier. And the CC foundation doesn't help because they haven't even conceived that kind of usage, let alone show usan example of how to do it.