 Solution

Quark mass differences break flavour symmetry. Isospin exchanges $u,d$, so its strong breaking is controlled by the small difference $m_d-m_u$. U-spin exchanges $d,s$, so its strong breaking involves the much larger $m_s-m_d$. Relative to the characteristic strong-interaction scale, the latter is much larger; this explains why U-spin is a substantially poorer symmetry.

Flavour-dependent interactions are a second source. Electromagnetism distinguishes $u$ from $d$ because $Q_u=2/3$ and $Q_d=-1/3$, and therefore breaks isospin. However, $d,s$ both have charge $-1/3$, so their electromagnetic couplings alone do not break U-spin in the equal-mass limit. If two breaking mechanisms for each symmetry are required, the second common mechanism is the weak interaction: the CKM matrix and weak flavour transitions distinguish the flavours and do not conserve either strong-interaction flavour symmetry. The dominant difference in the quality of the two approximate strong symmetries is still the mass splitting.