It's no doubt that sequels can easily cash on already existing publicity so naming them same with I, II, III is definitely an obvious choice. But there is more to it, before last couple of decades there weren't any cheap and fast mode of communication, there wasn't any internet,no rss feeds,no blogs, no push notifications so it's obvious that marketing had far less speed and reach so producers, distributors were a bit reluctant to "try something new" as far as sequel names are concerned because name matters! Also there was a limit on how much you can use traditional (newspaper, TV,cinema promos and promo events). Also for those "less-informed" crowd who weren't much into movie but nonetheless loved popular box office hits, a familiar poster with familiar title helps boost their confidence in the movie.
But unlike those days people these days are "beyond well" informed you can this very moment find out sequels scheduled for the next couple of years, naming your sequel something "new" and marketing that would definitely create a novel interest in people's mind which could later help push them to theaters.
But there may be another reason too it is the main subject of your movie if your sequel has something new, you can base your naming on that there are many examples: from Star Wars to Harry Potter and everything in-between.If your movie is all about sharks,dinosaurs naming them in a numeric order definitely makes sense, for instance take Jurassic park what do we see in the first movie? dinos! in second movie? city invading dinos? third one? flying dinos! What if they had named the third installment on "Isla Sorna", the island on which it takes place?
obviously a bad choice because there was no special significance of the island dinosaurs or the characters they were all almost the same. You see the point now naming sequel differently is effective only when there is a significant difference (or upgrade) in plot and/or characters. A great example (though not that great movie series) is Pitch Black (2000) it starts as a typical team survival on an alien planet but at the end only one significant character an antihero Riddick (Vin Diesel) survives due to this the sequels had diverged significantly in characters as well as plot and therefore named differently.
But what's really interesting about this case is that when it was re-leased on DVDs ,the original itself was renamed! (to The Chronicles of Riddick: Pitch Black) just to account for the plot differences.