The licence explicitly says: "The Free Software Foundation may publish revised and/or new versions of the GNU General Public License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns."
A non-copyleft licence would not be similar in spirit.
I could certainly imagine some proprietary code loving coalition seeking to control the FSF after RMS's eventual demise, so that they could create a new, non-copyleft licence.
GPL 3 is aware of such concern: it says, "If the Program specifies that a proxy can decide which future versions of the GNU General Public License can be used, that proxy's public statement of acceptance of a version permanently authorizes you to choose that version for the Program", apparently trying to provide an alternative path to allow Linus Torvalds to accept only the versions he wants to accept.