I would imagine that there are three contributing factors: weight, overall length, and dispatch intervals. Weight is surely the easiest to consider - I don't know how much each car weighs, but they don't look light. As
@Jodon mentioned, RMC is probably balancing the intensity of their rides and the elements they select with the weight of the train. Overall length is another likely factor that probably doesn't pop into people's heads. Many of RMC's rides are conversions and have pre-existing infrastructure that acts as a constraint in design. Again, I don't know the length of each car in a train, but it's probably not identical to the PTCs or whatever rolling stock the previous ride had. I also cannot find a predecessor that used more than seven cars per PTC 2-bench trains. Finally, RMC may have realized that a six car train is as optimized as can be for hitting the best dispatch interval possible, so that's just what they do on every ride.