Question

Why are the quarks and leptons in the first column stable but the second are not?

Answer 1

The leptons and quarks in the first column of the standard model are also called the first generation particles.

The first generation leptons are the electron and the first generation quarks are up and down quarks.

The electron is stable because it is the lightest leptons with electric charge - e. So it cannot decay to a heavier lepton  because that would violate the mass-energy conservation.

Similarly, up and down quarks are the lightest quarks with electric charge 2e/3 and -e/3. So these quarks cannot decay to heavier quarks because that would violate the mass-energy conservation.