That's an interesting question! I think it's a combination of things. Firstly, the ship sank on an even keel, and took 2 hours 40 minutes to sink. Most shipwrecks of the time listed badly and sank quickly: for example, the Empress of Ireland, which sank in 1914 with a greater loss of passenger life, went down on her side in 18 minutes. The Titanic's stability and longevity allowed many dramas to play out, and the (relatively) large number of survivors allowed the stories to be told.

Then there's the Greek tragedy element. If only they had have seen the 'berg 30 seconds sooner or 30 seconds later. If only one less watertight compartment had been breached. If only the 'Mystery Ship' (likely the SS Californian) had come to their aid - although in all fairness, it may not have made a huge difference. If only the ice field hadn't have stretched so far south that year. If only the radio messages warning of ice had been heeded. If only the Californian's radio operator hadn't switched off his set and gone to bed, just before the collision. If only Captain Smith had turned 'the Corner' half an hour later, to put the ship on a more southerly track. If only ... you get the idea.

Then there's the conspiracy theories and the associated mysteries. Was the Mystery Ship really the Californian? Was the Titanic swapped for RMS Olympic and sank in an insurance scam? (No.) Was a cursed Egyptian Mummy in the hold? (No.) Did she hit the ice along the side of the hull or the keel? Or both? What about the book 'Futility' published years earlier that seemed to predict the disaster? Did an Officer really shoot a passenger? (Very doubtful.) Why did Lightoller insist on women and children ONLY in the lifeboats rather than woman and children FIRST? Is the the Rubáiyát of Omar Khayyám on the wreck and can it be retrieved?

Then there's the historical elements. The Marconi radio set is the only one left in the world - why can't we be allowed to salvage it? Few photographs were taken of Titanic: most extant photos are of the RMS Olympic, one of her sisters. How many blades are on the central turbine propeller? How does the ship differ in design from her sisters? Why did so few Third Class passengers attempt escape before the lifeboats were launched? What songs did the band actually play? (It probably wasn't Nearer My God To Thee.) Is the swimming pool still full of water? etc. etc.

Then there's the people: the last of the Gilded Age billionaires and socialites, thrown together with the Second Class 'ordinary' folk and the Third Class immigrants from all of Europe. How did they act and react? Who accepted their fate with dignity and tried to help their fellow passengers? Why did J. Bruce Ismay get into a lifeboat knowing full well how he'd be treated if he survived? Were the crew of Boat #1 actually bribed by the Duff-Gordons not to fish out survivors? And so on.

I'll stop here before I get carried away.

Edit: typos.