On the official PlayOverwatch webpage, before the game launched Blizz made cosplay reference kits made publicly available with high quality source images that detailed shapes, special logos, and even color codes.
My guess would be that it just takes too much time to do on top of everything else that comes with adding a new hero, and so they decided it wasn’t worth it. It’s a shame, but maybe they’ll revisit it someday.
I don’t think it would take that much time away from new hero releases. It’s just screenshots from concept art and in-game renderings of models on transparent backgrounds, placed on a pre-designed artbook-esque page.
They aren’t making beauty render for promotional materials, and they’ve encountered the color codes and A-poses a couple thousand times while they were making the character art.
Judging from the existing reference kits that follow an established layout/template, they clearly had the tools set up for it to support future reference kits.
Honestly it feels like they just forgot it about it. Which, I could totally see them getting swept up by all of the OWL skin making and event stuff that they forget.
Side note, OWL skin reference kits would be really cool…