#### Answer

- Because the pH and pOH values would be, almost always, negative, that could make the calculations a little bit harder.
- If we look into the approximate value of $K_w (25 ^{\circ}C)$: $10^{-14}$, we will see that this value is easily represented by a logarithm with base = 10, if compared to a natural one ($ln$), where the is base = "e".
Besides that, the "e" is chosen in other chemistry studies, because the equations are related to integration/derivation. If it doesn't have this origin, the logarithm that is the easiest to work with is the one with base 10.

#### Work Step by Step

- Concentrations ($M$) are, most of the time, represented by number that are lower than 1.
All the logarithms have this propriety that, for numbers between 1 and 0, the result is negative. So, when we put a "-" in front of "$log[H^+]$", we are making the calculations easier, by turning it to positive again.
- Using the base 10, for example, we can say that acidic solutions have a pH < 7. But, if we used "e" as the base, we would say that: acidic solutions have a pH < 16.18809565, so it is clearly easier to use the base 10.