There aren't many women or minorities in racing because there just aren't many women or minorities who race. It's that simple. It's a numbers game. It's the same reason why there aren't many African Americans in baseball. Most young American black youths generally choose football or basketball over baseball. Meanwhile, baseball is the most popular sport in countries like the Dominican Republic, which is why there are so many in MLB. Same reason why there's so many Canadians and Russians in hockey. Same reason why US men's soccer is generally awful, because our best athletes usually choose other sports. It's all relative.
Look at where racers come from. Look at anyone that advances past go karts, quarter midgets, and late models on the local tracks. The vast majority are white males, and only a very small percentage of these racers will ever make it to NASCAR. So it's not surprising that there's a "lack of diversity" because most folks interested in racing careers are statistically white males.
If we can get more women and more minorities interested in racing, it'll take care of itself in time. The cream will always rise to the top, and talent won't care about your gender or ethnicity.