I think if you look at it through the lens of a non-coaster enthusiast, or a non-amusement park junkie, most would never know where any of those parks are. so to say the parks are underrated, they may very well be to the GP.... As for Six Flags, it could be in the branding. Their are national commercials for Six Flags (or there were). So to the GP in a Six Flags area, they see a national ad and think "wow they are talking about my park, its a national destination." Now compare that to the Cedar Fair chains, each commercial is park specific. So it feels more regional even in an advertising sense....
True story, no lie. My family is a huge amusement park family. My mom has been going to KI since it opened and Coney before it. She got us KI passes when we were younger, etc. She never knew Holiday World existed in Indiana or that it offered such a great experience. When I talk about getting a Platinum Pass for my wife, son, and I, my extended family (and wife) always mention going to CP, Kings Dominion, and KI. They never knew about Carowinds or other parks in the chain.
Sometimes, I think since we have our fingers on the pulse of the amusement park community we forget how regional parks truly are...