I'm left puzzled by why they felt the need to do this.

I suspect the answer is something like "those materials weren't available and we had to improvise."

Anime (and Japanese film in general) has a horrible archival track record, even for beloved titles like these, so decay might have been a factor. Alternatively: is there a decent anamorphic R2/Jp release of this with the credits sequence intact? Will there be in six months? R1 licencees have often received substandard or incomplete materials if the R2 release was still pending.