The other reason more Six Flags/Paramount parks are not on this list is because they are mostly seasonal parks, while Disney parks, Universal Parks and most Busch parks are open yearly.

The Seasonal parks are bolded:


Magic Kingdom at Walt Disney World in Orlando, 16.1 million, +6.5 percent
Disneyland in Anaheim, Calif. 14.5 million, +8.5 percent
Epcot at Walt Disney World in Orlando, 9.9 million, +5.5 percent
Disney-MGM Studios at Walt Disney World in Orlando, 8.6 million, +5 percent
Disney's Animal Kingdom at Walt Disney World in Orlando, 8.2 million, +5 percent
Universal Studios Florida at Universal Orlando, 6.1 million, -8.5 percent
Disney's California Adventure in Anaheim, Calif., 5.8 million, +3.6 percent
Universal's Islands of Adventure at Universal Orlando, 5.76 million, -8.5 percent
SeaWorld Orlando, 5.6 million, +0.2 percent
Universal Studios Hollywood, 4.7 million, -6 percent
Adventuredome at Circus Circus in Las Vegas, 4.5 million, +2.3 percent
Busch Gardens Tampa Bay, 4.3 million, +5.1 percent
SeaWorld San Diego, 4.1 million, +2.5 percent
Paramount Canada's Wonderland in Maple, Ontario, 3.6 million, +7 percent
Knott's Berry Farm in Buena Park, Calif., 3.47 million, -3 percent
Paramount's Kings Island in Kings Island, Ohio, 3.3 million, -5.1 percent
Morey's Piers in Wildwood, N.J., 3.1 million, +1 percent
Cedar Point in Sandusky, Ohio, 3.1 million, -2 percent
Santa Cruz Beach Boardwalk in Santa Cruz, Calif., 3 million, flat
Six Flags Great Adventure in Jackson, N.J., 2.9 million, +6 percent
Six Flags Great America in Gurnee, Ill., 2.8 million, +24 percent
Six Flags Magic Mountain in Valencia, Calif., 2.8 million, +5 percent
Hersheypark in Hershey, Pa., 2.7 million, flat
Busch Gardens Williamsburg, 2.6 million, +8.3 percent
Dollywood in Pigeon Forge, Tenn., 2.3 million, +7.3 percent
So being that PCW is only open 3 or 4 months a year I think it's pretty impressive they got 3.3 million people.