Probably for the same reason for which I sold all my original valiant books (mostly in NM) for about $0.50-1.00 each about a year ago?
I think you answered your own question... a lot of people (particularly dealers) thought they had tons of unsold Valiant books that were worthless, but when they went to look, they couldn't find the early Valiant keys that got hot.
Valiant collectors have known for years that dealers THINK they have lots of Valiant, and they often laugh when you ask about Valiant... but you actually start looking at their inventory and they have Magnus starting at #17, X-O starting at #13, etc.
And the reason is...................there were a few of us that started cleaning them out of pre-unity, last issue, and gold/platinum/VVSS books at 3 or more for a dollar back in the 1999-2002 period.
I do regret on passing on the longboxes of Rai #0 now, though.