Why is the isoquant curve down sloping?

The isoquant curve is down sloping because it represents the trade-off between two inputs, typically labor and capital, while maintaining the same level of output. As one input is increased, the other must be decreased to keep production constant, reflecting the principle of diminishing marginal returns. This negative relationship indicates that to substitute one input for another, a firm must reduce the quantity of one input to maintain efficiency and output levels.