Why tea is grown in hilly regions mostly
Answers
The suitable climatic conditions for the crops mentioned in your question are happens to be found only in Tropics and at hilly areas (only for Tea and Coffee).
There are plantations like Sugar cane which grows very well in sub-tropics region, for ex: UP state in India. Many plantations are also grown on levelled surfaces, for ex: Jute, Coconut, Oil Palm, Cashews etc.
In case of Tea and Coffee crops, temperature and abundant rainfall are one of the requirements at various stages of crop growth. There shouldn't be water stagnation as well. This is easily possible in hilly areas located in tropics.
Cashew and Oil palm crops are also Tropical crops which depend on temperatures that were possible mainly in Tropics. These don't need to be grown on hilly areas if adequate water and suitable soil conditions are available on levelled surfaces.