The grain production heavily depends on how big of a consumer of grains a country is, and how large the population is, as well as the demographic trends in the country or region.
In this image, we can see that the USA and China has been experiencing very big growth in the production of grains for a long period, with the increase declining in more recent times, though it is still the largest increase. The reason for this is that both countries had large and quick growing population, also being large consumers of the grains as large portion of the daily food is based around them. As the growth of the populations started to decrease, there's also a decline in the increase of the grain production.
Europe doesn't have very large population as a continent, and it has also kept the number of the population relatively stable. After having initial growth, its population has been stagnating and even declining, which has resulted in an initial slight increase of the grain production, and than a decline in it. Also, it is not the whole continent where the grains form the main dishes of the daily consumption of food, thus the production has not been so big.
Africa has had a very low, insignificant growth in the grain production. Despite the continent having an ever growing population, as well as suffering from malnutrition, the way of the agriculture practices are still primitive, and the production has remained low.