Actually SOLAS banned ships with wooden superstructures from overnight passenger service back in 1966. The Delta Queen had been running on an exemption granted by the US Congress since then, but when that exemption came up for renewal in 2008 (as it had done every few years and was always routinely passed), it was blocked by the chairman of the house transportation committee because under the current owners, Majestic America Line, the crew was non-union.
The Mississippi Queen was built in the mid-70's, and has a metal superstructure. She was undergoing a major refit when Majestic America Line decided to get out of the Mississippi steamboat business after the 2008 season, a decision that was presumably a combination of the Delta Queen losing her exemption and the slumping economy. Most of her interiors were gutted, and worse, she had become infested with toxic mold during the layup in New Orleans. There had been some talk of her becoming a hotel somewhere along the river, but considering the amount of work she would have needed just to get her into a usable condition even for a static role, plus the fact she's not particularly historic, not to mention the ongoing recession, apparently the cost proved to be prohibitive.