Since the LP was then on all 50 state ballots and Anderson was not, the National Organization of Women (which sponsored the debates) couldn't come up with any rational metric which would have allowed Anderson to participate while excluding Clark.
Actually, it was the League of Women Voters who sponsored the debate, not NOW.
There were two debates in 1980. John Anderson did qualify for the first debate and did in fact participate in it. The debate was September 21, 1980 in Baltimore and was moderated by Bill Moyers of PBS. Jimmy Carter refused to participate in that debate, because of Anderson's inclusion -- so it was only Reagan and Anderson who were present.
The League of Women voters did exclude Anderson from the second debate on October 29, citing a decline in his poll numbers below the minimum threshold.
Yes, my mistake.
But note also that Carter's team was initially asking for a 5-way debate to include Clark, Commoner, and Anderson.