Catalytic triads are seen in many proteases, with several having conserved residues. The protonation of these residues is important in allowing proteolysis, and residues that are protonated at physiological pKa values will be present here. Outside of this range of acceptable pH, the proteolytic activity of the enzyme will be reduced, as the catalytic triad will not be in the correct protonation state.
Inhibitors can target specific residues in the catalytic triad, permanently binding to them and preventing them from being active. This will reduce the activity of the enzyme, potentially completely suppressing the catalytic effect.