why not they bring out the sunk Titanic ship out off the ocean and keep it somewhere in a Museum

If you want something to put in a museum for people to explore, it would actually be cheaper to build an exact replica of it, than to develop the technology to raise something that big from that depth, transport it to shore, put it back together, clean it out, and restore it. Like others have said, the ship is in too bad of a shape to raise intact. It's broken into multiple pieces that have corroded and rusted into an unrecognizable state. Sea creatures have since taken it over, so raising it would destroy that habitat and kill a lot of things. Plus, there's that whole "it's a mass grave" thing that you don't want to disturb.