Pryor and Spolsky have been running Fog Creek for sixteen years. That's a log of 'everyday at the office' to go without a big payday, particularly considering how much StackOverflow could have been sold for years ago.
From a strategic standpoint, selling Trello is probably a more favorable way for Spolsky and Pryor to raise $360 million in cash than through venture capital. In hindsight it's not implausible that the goal behind developing Trello was always to sell it off. Unlike StackOverflow, Trello has lots of competitors and it's value in is in integrating with the rest of a project's workflow. Essentially it's a commodity.
From a strategic standpoint, selling Trello is probably a more favorable way for Spolsky and Pryor to raise $360 million in cash than through venture capital. In hindsight it's not implausible that the goal behind developing Trello was always to sell it off. Unlike StackOverflow, Trello has lots of competitors and it's value in is in integrating with the rest of a project's workflow. Essentially it's a commodity.