I would agree, too. He was a very solid artist (I have one of his pages), but he didn't generate the "ooo..." factor. He also did a lot of work, which keeps the price down.
One aspect of OA which I don't think gets raised much is the inherent tension between art which furthers a story and art which makes for a great display piece. Morrow's work is very good at moving the story along, and that Supergirl page undoubtedly did so. But, that doesn't rank highly in the OA market. Instead, people want splashy pages (pun intended). They rarely move a story along very well, but they do look great.