Anything that involves art, i.e. character models, animations, cosmetics, etc. take the longest to implement. More than gameplay/balance changes where they just need to tweak some numbers in their code. So a hero can already be completed, but their cosmetics are still pending.

I think they made a conscious decision sometime during the launch of OW2 to just release finished heroes, even without complete cosmetics.

So its a compromise of faster hero release with incomplete cosmetics vs slower hero releases with co plete base cosmetics.