Carowinds is surrounded by a market area of about 14 million people
King's Island's market area is about 15 million people
Canada's Wonderland's market area is around 9 million people
King's Dominion has a total market area of about 19 million people
Where are you getting these market numbers from, and what distance radius does a market consist of?
The immediate answer to your question that I can give is this: For starters, Canada's Wonderland IIRC has the highest seasonal amusement park attendance in North America. Now, you could kind of twist that, and point out that there's no need to add any new big rides (which is what they've been doing for the last couple of decades until Behemoth), since people will come regardless.
14 million is a lot of people, and I'd like to see where that comes from considering that the entire population of North and South Carolina combined is just under 14 million (I guess that is where you got it?).
Here are the numbers that I would say REALLY matter for regional parks like these:
Charlotte metro population: 2.5 million
Toronto metro population: 5.5 million
Cincinnati metro population: 2.1 million
Richmond metro population: 1.1 million
San Francisco metro population: 7.3 million
With that said, after Cedar Fair took over, Scott Anderson said that the park was seen as a prime opportunity for the company considering the high growth area, and to be patient. Maybe in 2010 that patience will pay off. Maybe not... We'll see..