Strangeness selection in pion-proton and kaon-proton resonances

The ${\pi }^{+}p$ channel has baryon number one, strangeness zero and $I_3=3/2$, matching the ordinary $uuu$ Delta baryon. The $K^{+}p$ channel has strangeness plus one, which no three-light-quark baryon can have because each strange quark contributes minus one. A resonance in that channel would require exotic content such as four quarks and an antiquark; the ordinary low-energy qqq multiplets therefore explain the contrast.