Answer: Because of the dense equatorial forest, heavy rainfall, poor transport and lack of basic facilities in the basin.
- The Amazon basin has a hot and humid equatorial climate with very heavy rainfall and dense evergreen forest, which is difficult to clear for farming.
- Transport is mostly by river, as roads are few, and malaria and other diseases are common.
- Fertile land is limited and shifting cultivation supports only small groups, so only a few towns and settlements have grown along the rivers.