Well, that's what happens when you don't look after a ship. She may have been tough, but father time is relentless and to borrow a phrase from Neil Young, "rust never sleeps".
I'm sure that the museum knew that the ship was in peril from neglect over the years but for whatever reason were unable to raise the money to at least protect her from the ravages of corrosion and time. It's a shame considering to whom she was named after.
She could be refloated I'm sure, the question will be whether or not there is the political will to repair her.