Because in soil, the acidic salts remain in the soil more so than in hydro setups, causing a lower-pH situation. Thus, soil grows require a higher pH going in.
Note that each number on the pH scale is an exponential order of magnitude. A pH of 5 is 10 times more acidic than a pH of 6. A pH of 5 is 100 times more acidic than a pH of 7.