Why are there always opportunity cost when we shift from making one product to another?

Because Some resources are better suited for use in making the first product.

In order to shift to other product, we need to sacrifice a certain amount of benefit that came from producing that product.

For example, when we decided to shift from using oil energy into solar energy, we had to sacrifice the low cost, the efficiency, and the current reserves of oil product and gaining the ability to preserve the environment.

There is always cost involved to various degrees when there is a shift from making one product to another because there are changes in production lines, employee knowledge/training, and marketing strategies. It takes time to change product production, and time always equals money.