Possibly because えい is the modern way of writing the long え sound. Basically, べえ (bee) and べい (bei) sound virtually identical, but the former doesn't follow modern Japanese orthographic convention.
Interestingly, it appears the original kana version of 半兵衛 was actually "ハンベヱ" using the deprecated ヱ "we" character, which had long since been pronounced and replaced with "え" anyway.
On the other hand, although ENAMDIC lists both "はんべえ" and "はんべい" as acceptable kana versions, all the native Japanese sources I see seem to prefer "はんべえ" including the character's tag's page on Pixiv. Perhaps we should consider changing / aliasing here?
Qualified the Nobunaga no Shinobi character. Aliased the Oda Nobuna character. Also did: create alias takenaka_hanbei_(sengoku_basara) -> takenaka_hanbee_(sengoku_basara) create alias takenaka_hanbei_(sengoku_musou) -> takenaka_hanbee_(sengoku_musou) create alias takenaka_hanbei_(musou_orochi) -> takenaka_hanbee_(musou_orochi)
And the OniAi character as well.
I am sort of iffy on changing romanization for archaic forms in general, and particularly when it's a modern character who shares he name, but I guess it's reasonable to do.