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
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.