I think it also was due to the simplicity of marketing a sequel with a number. Not just posters and trailers but even theater marques. I have seen non numbered sequels still being listed that way on theaters' huge street signs to save space.
I think with years of coverage in the build up to release its different now. More people are familiar with unique titles again. But through the 80s it became the standard casual film goers where confused by anything else. I remember in the early spring of 1992 wearing a "Batman Returns" t shirt. So it was a few months before its release. The imagine of Batman was clearly of Michael Keaton. Yet I had a few friends ask me if was for "Batman II". Which surprised me because I thought that title made it obvious.