The difference between "don't you know enough about it to say" and "you don't know enough..."?

Here, "don't you know enough about it to say" means "you don't know enough about it to say." What is the difference in nuance betwwen the two grammatical structures?

Context:
In 2004, the distinguished Gallup organization posed the following question to a statistical sample of Americans: "Do you think that (1) Charles Darwin's theory of evolution is a scientific theory that has been well supported by evidence, or (2) just one of many theories and one that has not well supported by evidence, or (3) don't you know enough about it to say?" Only one-third of Americans indicated that they believed the theory of evolution was well supported, with the remainder being equally divided between those who argued it had not been well supported and those who just didn't know enough to say.