Pretty much because as far as it goes, it's a movie based on historical events and not the first movie to feature those events. Sure, he created some characters like Rose and Jack to add a romantic flair to it, but the core of it is what makes it timeless. I think its success is a testament to the harrowing events those people went through and the fact that it's just a fascinating story, period.
Like all his films, it's also an absolute technical masterclass in FX and cinematography. The ship going down is still a stunning sequence that has barely dated from a technical standpoint. It's stuff like this that help make his movies so special.