> why wasn't it done before?

I saw (fenced-in) goats trimming grass in the Berkeley Hills as far back as 2000. I'm sure people were doing it long before then.

> why isn't this done more often

Possibly because it's hard to make sure than none of the goats escape, breed, and wreak havoc on the native species. Eradicating invasive species can be unbelievably expensive and brutal (e.g. helicopters and machine guns), even on small islands. Hopefully they've embedded GPS trackers in all of their goats, but I still suspect that they're taking chances.