Other methods have been used in the meantime, for decades. But they are only so effective. Vitamins, other foods, and other methods have been in process. But they each have their own limitations, both on supply to remote areas and getting local peoples to take up those methods.
The latter is the biggest issue, especially with trying to introduce alternative foods like carrots. If they aren't a part of the local cuisine, many of the individuals, who are often subsistence farmers who have limited land and only grow explicitly what they need to survive, aren't interested.
Hence why golden rice was developed, because rice is a main part of the local diet in these areas and so it is much easier to get them to adopt growing a different cultivar of something they already eat than it is to convince them to grow a completely different food.