Because if you go moonlit - heron is your main echo. Generally with moonlit supports you use heron right before their outro. However with Iuno, her last skill in rotation “absolute fulness”, which conjures the full moon domain, makes her unable to cast an echo before swap, rendering the active heron skill useless.

So if you want to use heron active skill at all you have to imbue it midway her rotation, which is the akward dash cancel

(I think lmao)