The answer is that Ross Andru penciled most of a terrific-looking 15 page insert to be included in Swamp Thing, before going off into her own book. But for reasons sadly lost to history (meaning, for the life of me, I can no longer remember why), we never went ahead with the series.
Ross Andru created most of an attractive 15-page insert for Swamp Thing but then began work on his own book. The reason why this series was never completed remains a mystery (I can no longer recall the specific reasons).