Why did biologists in the 1960s recommend reintroducing wolves to Yellowstone?

One of the main reasons why biologists recommended we add wolves to Yellowstone was that by adding them, more biological diversity could be added to it. This as a consequence meant that the natural ecosystem would be more robust and able to withstand challenges in the future.

I believe the answer is: Wolves could control the increasing elk population

At that time, the increasing elk population became a trouble for Local Farmers because they eat and destroy the farmers' crops which cause many financial expense for them. Because of this, biologist introduced wolves as the elk's natural predator to reduce elk's population.