Answer:
If that crop fails so would the agricultural production of said nation which could lead to famine or economic breakdown
Step-by-step explanation:
When you rely on only one crop then your basically sitting on a ticking time bomb. If the crop fails and you don't have a replacement, your agriculture is disabled. If that crop is something like cotton, You loose your main export and ability to employ people. If its something like Rice then your citizens loose food which will cause famine in the lands which could lead to rebellion.