Because Gell-Mann could find in the Baryon-Meson-SEXTETT (Neutron-Proton-Pion-Antineutron-Antiproton-Kaon-Neutron) only 6 Triangles. And their Points of Gravidity are the 6 Quarks, where each Triangle with the opposite Triangle represents a Quark Family.
Therefore the classical Particle Physic knows only 3 Generations of matter.
The gray matter Physik may find more.