Answer
Because tap water has high concentrations of ions such as Na⁺, Ca²⁺, Mg²⁺, Cl⁻, .. etc which conduct electricity.
Work Step by Step
We differ between distilled water that is extremely weak electrolyte as its ion-product constant is very small and equal to 10⁻¹⁴ so distilled water does not conduct electricity, and tap water which its conductivity resulting from its content of ions.
