The reason is it's the closest amusement park for a lot of people.
Next closest park to PKI (from "Cincinnati, Ohio", using Google Maps):
Six Flags Kentucky Kingdom (Louisville) - 2 hours
Holiday World (Santa Clause, Indiana) - 3 hours
Indiana Beach (Monticello, Indiana) - 3.5 hours
Cedar Point - 3.5 to 4 hours
Erieview Park (Geneva-on-the-Lake, Ohio) - 4 hours
Geauga Lake - 4 hours
Six Flags Great America (Gurnee, Illinois) - 6 hours
Six Flags St. Louis - 6 hours
Paramount's Kings Dominion - 9 hours
Busch Gardens Williamsburg - 10 hours
And yes, Cincinnati is in a great spot:
1 hour from Dayton and Wilmington
2 hours from Indianapolis
2 hours from Lima
2.5 hours from Portsmouth
3 hours from Marietta
Within 3 hours of several other small cities/towns (Wilmington, Chillicothe, Manchester, Georgetown, Washington Court House, Ripley, Russleville, Maysville, Kentucky, etc.)
4 hours from Pittsburgh