the reason of why titanic being so beloved until now, is that, no major epic love story being released since 1970. The last big scale melodrama since titanic was doctor zhivago during 1965, since then, Hollywood were so eager to produce sci-fi and action blockbuster, like star wars trilogy, indiana jone, ET, back to future, batman, jurassic park, 007 and so on and so forth. Those blockbuster was aimed to meet fan boys's demand. The girl's needs was completely ignored by the time.
When titanic released in 1997, it combined an old-fashioned tragic love story and spectacular visual effect, the hybrid to approach adopted by James Cameron was extremely successful as US and audience across the world are eagerly thirsty for a new generation iconic couple, which was unseen for more than 3 decade.
Even until now, when we talk about the iconic couple on the sea, the general public will still talk about the tale on titanic, not to mention that immensely popular themed song-titanic.
The titanic's success is largely attributed to the girl's power, that's not much films that filled with feminist element able to storm box office.