Because the yams in the traditional African diets aren’t doing the trick.
Because so much of Africa is arable rice growing land.
Why rice? Why not sorghum or teff. Those are grown in much of Africa.
There's not a lot of vitamin A in yams, so maybe GR2 is a better the option.
Why rice?
Uh, because rice is eaten and grown in more than a hundred countries, and it is the staple food for more than 3 billion people. When you look at the caloric intake of these people, rice provides 60%-70% of their total nutrition. These same people also happen to be the most affected by vitamin A deficiency. So, yeah, rice is a logical and exceptional choice. And given the resistance it has met by people who think it's better for their flagrant ignorance to prevail rather than for people to live, it's no wonder these companies aren't rushing to do the same for sorghum, teff, or any other product.
You should be asking that question to a mirror.