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It is because the U(2) x U(2) symmetry breaking is much smaller than the U(3) x U(3) symmetry breaking, not to speak about U(6) x U(6) symmetry which is badly violated by large masses of heavy quarks (c,b,t). The characteristic scale of strong interactions is 1 GeV (the proton mass), while the u,d quark masses are of order a few MeV. Thus the predictions of U(2) x U(2) symmetry for hadrons must be valid to an accuracy of about 1%. The s-quark mass is much larger (around 100 MeV), so U(3) x U(3) symmetry predictions are not nearly as accurate. Of course, chiral anomalies destroy the axial U(1) symmetry, so we should really be talking about SU(2) x SU(2) x U(1) instead of U(2) x U(2) and SU(3)x SU(3)x U(1) instead of U(3) x U(3).