Every time people ask about things like this we hear excuses like that it's difficult to do or that it takes time...
They had six years, and the game has generated them more than $1 billion in lifetime revenue.
They have not made it, because they did not want to make it. Think about that, the next time you buy their 30th pack or watch their marketing ads. They did not want to hire any artists to fix their skin tones or add more of them, but they happily hired people to make more packs to sell, or make unique animations in their marketing trailers.
I know I'm getting pretty tired of hearing excuses.