Rubber plantations are predominantly found in South India, particularly in Kerala and Tamil Nadu, due to several key factors:

### 1. **Climate**
Rubber trees (Hevea brasiliensis) thrive in warm, humid tropical climates. The regions of Kerala and Tamil Nadu provide ideal climatic conditions, including:
**Temperature**: Rubber requires a temperature range of 25°C to 35°C for optimal growth.
**Rainfall**: These states receive ample rainfall (1,500 to 2,500 mm annually), which is crucial for the development of rubber trees.

### 2. **Soil Conditions**
Rubber trees prefer well-drained, loamy soils rich in organic matter. The lateritic soils found in many parts of Kerala and Tamil Nadu are suitable for rubber cultivation, allowing for healthy tree growth.

### 3. **Topography**
The hilly terrain of the Western Ghats in Kerala and Tamil Nadu is conducive to rubber cultivation. The slopes facilitate good drainage and prevent waterlogging, which is essential for rubber trees.

### 4. **Historical Development**
Rubber was introduced to India in the late 19th century, and Kerala was one of the first states to adopt rubber cultivation on a large scale. The established infrastructure and expertise in rubber farming have led to its expansion in these states.

### 5. **Economic Factors**
The rubber industry has become a significant source of income for many farmers in Kerala and Tamil Nadu. The presence of processing units and cooperatives has further incentivized rubber cultivation, making it a viable cash crop.

### Conclusion
In summary, the combination of favorable climatic conditions, suitable soil types, topography, historical factors, and economic incentives has made Kerala and Tamil Nadu the primary regions for rubber plantations in India.