No one decided to make it grass, it's always been grassland. Here it is in 1948, when it was farm land. Then they used it for a rock quarry, which made a real mess of it, and was never cleared up properly. The trees are a relatively new feature. Much of it was farmland up until the late 60's. Most of the trees on the west area didn't appear until the early 80's.
But some strategic planting of native species might be a good idea, make some more groves, leave some grassland...