Grumpus wrote:
... just wondering if I missed an explanation.
No, I don't think you're missing anything. There are no published, official reasons why surviving Roseguard members left the keep's dungeons alone. It does seem counter-intuitive to our own adventuring
The Adventure Toolbox says that, "Grieving their lost friend, the Roseguard left, sealed up the keep, founded what would become the town of Otari, and retired there. They never knew of the keep’s lower levels or the many creatures still remaining within them" (Abomination Vaults, page 75).
I imagine they saw the keep as a tomb, the grave of their friend, Otari. There may have been a conversation about respecting his grave site: they never did recover his body. And as it notes above, they "sealed up the keep." We don't know what that means, but it sounds like they made an effort to keep what was below ground trapped in the depths - which they accomplished. Never knowing something was below the swamp isn't the same as not suspecting. And, they stayed in town, perhaps keeping a silent and life-long vigil on the keep. Nothing ever bubbled up from the depths and the Roseguard survivors died in peace, always maintaining respect for Otari's final resting place.
I am rambling, but there ya go - another take on it.