3 x 7k axle but only 20k GTW???

Looking at upgrading TT toyhauler for 5th wheel toy hauler (grand design momentum) specific floor plan still being debated. Anyhow, why... if it has three axles rated @ 7k each (not to mention pin weight) would it only be rated for 20k max? What is the weak link; pin, frame, hangers, other???