Question

Why was agriculture the primary concern of the kings?????

Answer 1

Because most of the benifits like...free food items up to some extent, more tax, etc. will be taken from the farmer as he cultivates crops. And when in the extreme conditions, all of the people, includind the soldiers, and also Kings depend on the food items preserved.
And every farmer has to pay the tax as money and the food items he cultivated and the products he produced
Hope this is better. You can give it the best.

Answer 2

Because agriculture is one of the way to get food for him as well as for his people. And without food it is not possible to survive.