I don't think it's a question of the materials not being available. Not only did Manga's earlier, non-anamorphic release of Cagliostro several years ago have the opening credits in their complete kanji-covered glory, there's been a Japanese R2 anamorphic version that had them since around the same time. (The only reason the original Manga Cagliostro release wasn't anamorphic is that Manga was basically lazy about it back then, and didn't put out any anamorphic discs unless an anamorphic transfer was dropped in their laps, as with the original Ghost in the Shell.) There was nothing stopping them from including it if they'd wanted to.

I expect that what "wasn't available" was a clean, kanjiless version of the opening credits, it's been so long since the movie was made—so rather than go with the existing ones, they decided to go with screenshots instead. (Some discussion elsewhere suggests that Tokyo Movie Shinsha, the Japanese studio that made it, insisted on having English-language credits at the front. Still, Manga could have overlaid them over the existing ones with subtitles, or put the existing ones in an alternate angle. Bah.)