Because it's been sitting in only 30 feet of water for 150 years. Metal rusts.
“...Metal rusts...”
I know...
But - there’s probably still a lot it intact, enough to raise and maybe attempt a restoration.
Hell they pulled a busted up WWII P38 Lightning out of 75 feet of solid ICE up in Greenland several years ago. The ice had started crushing it.
Anything is possible if you want it badly enough. This is a piece of American history.
Then again, it is also a tomb... maybe it IS better to let it rest.