What they said
https://ja.wikipedia.org/wiki/ネーム_(漫画)
ネーム is specifically just used for manga, to a point where the author adds a note in the manga itself to explain its meaning.
Had ネーム been used in other fields as well, then we would have went with translating it as "storyboard", but since it's such a specifically manga term, we felt it more appropriate to stick to using "name" instead.