The Lord protected his vineyard by fencing it, and he gathered out the stones, or evicted the wicked previous tenants. He planted it with the finest, choicest plants (his covenant people of Israel; see also Jeremiah 2:21). The Lord does all he can, but moral agency is guaranteed; some choose to be wild.
What more could have been done? (The same question is asked in Jacob 5:41.) We have a courtroom scene again; this is a pleading question.