Several reasons:
1. It was the ship's maiden voyage
2. She was the largest ship in the world at the time
3. The number of people that died, making it the worst maritime disaster in history up to that time
4. She sank slow enough that there was TONS of drama: the Strausses electing to stay together on the ship knowing they would die, Benjamin Gugenheim and his valet wearing their finest clothes and declaring, "we are dressed in our best and prepared to go down like gentlemen," John Jacob Astor, one of the richest men in the world at the time, dying after placing his pregnant wife in a lifeboat, the band playing until the very end, finishing with Nearer my God to Thee (haunting as hell in my opinion). The list goes on.
5. Believed (at the time) to be the first use in history of the distress signal SOS which had just replaced the previous call, CQD.
6. The fact that the Captain was supposedly on his final voyage before retiring (there are documents both supporting and disproving this)
7. The dramatic rescue by Carpathia, rushing to Titanic's aid and nearly hitting several icebergs herself to try and save lives
8. It was seen as the ultimate expression of how a man was expected to act on the Edwardian era: women and children into the boats, the men stoically staying behind to accept their fate, the crew remaining at their posts until the very end, knowingly sacrificing their lives to keep the pumps running and the power on
9. The fact that the press had billed the ship as "unsinkable," yet she sank on her maiden voyage (note, neither White Star nor Harland & Wolff ever made that claim)
And those are just off the top of my head