As a person with literally 3 or 4 pieces of OA (of any book) - I can see why possibly.
Perhaps most 'fans' and not "collectors" just want one, maybe 2 pages.
If so, then there was plenty to go around, in the shape of 'minor' pages.
Collectors duke it out for covers and splash pages, TM and key 'killing' pages.
Perhaps, and this is only my , the little guy (like me) is happy with 1 or 2 pages, and the pool for 'minor' stuff is dwindling?
$630 though, is still pretty good for a page that was what, $175 - $225 max, back then?
I can't remember exactly what I paid for my single (maybe $140?) or my double ($400?) but both have top tier characters in, and both, based on the sale of this page, represent a decent profit should I decide to sell.
I'm not - I've had them framed etc, but if you bought that page for $150 - $200, you'd be pretty happy with $630 surely?
There is nothing special about that page.