Question #d19cf

Because that is how the $p H$ scale is defined...........
$p H$ stands for $\text{pouvoir hydrogen}$, the power of hydrogen, based on a logarithmic scale. The hydrogen ion was conceived to be ${H}^{+}$ or ${H}_{3} {O}^{+}$, and $p H = - {\log}_{10} \left[{H}_{3} {O}^{+}\right] .$
In the days before calculators, multiplication and division of large and small numbers was simplified by the use of logarithms, and standard tables of logarithms to the $\text{base e}$ and to the $\text{base 10}$ were widely published for use by scientists, and students, and engineers, and also no doubt accountants.