The Empire Strikes Back didn't have a "II" in its title, either. Except before anyone knew what it was going to be called, no one called it Star Wars II, and once it came out, no one called it Star Wars V, or even Star Wars Episode V. It was always The Empire Strikes Back or just Empire for short.
A decade earlier, there was no Planet of the Apes II, either. The Ape sequel was called Beneath the Planet of the Apes.
James Bond II? No. From Russia with Love.
And so forth.
At that time, there was no hard-and-fast rule that sequels had to have "II" or "2" in the title. I don't think you can make anything out of not having it, except that they chose not to put it there. They might have just thought it sounded classier without it; it does, by the way.
Yeah, the whole Roman numeral thing is a relatively recent convention, Hollywood-wise. Back in the day, sequels had titles, not numbers: Bride of Frankenstein, Tarzan Finds a Son, Sherlock Holmes and the Voice of Terror, Return of the Thin Man, Topper Takes a Trip, Godzilla versus the Smog Monster, Taste the Blood of Dracula,
I suspect it was the Godfather
movies that started the trend of numbering sequels, possibly because it seemed classier.