It's a poll of registered Republicans and independents. Still, your point is somewhat valid. Why don't they do likely voters only? You'll notice that up to 22 percent of these people have never heard of some fairly well-known candidates--not exactly the most well-informed bunch.
Sorry, it's even worse than that: 42% have never heard of Mitt Romney, and 58% have never heard of Chuck Hagel.
No matter what sample size, or who they "poll", it will be biased towards someone CNN prefers, and biased against someone CNN wants to see go away, because of their agenda.