OR because labor and capital inputs are used more effectively. This implies an increase in technology and innovation. Productivity measures how effectively an economy uses its labor and capital inputs.
Output is produced by combining inputs of capital, labor and land.
Technological progress enables an economy to increase output with the same amount of inputs. However, note that capital and labor share in production. If capital is growing faster than labor then capital accumulation contributes too. The first tells us how much capital each worker works with. The other tells us how much output is produced per unit of capital.