Answer
Because stronger acids and bases would completely dissociate in water.
Work Step by Step
If we have some acid, $HA$, with $K_a$ greater than $H_3O^+$, this reaction will completely occur:
$HA(aq) + H_2O(l) -- \gt A^-(aq) + H_3O^+(aq)$
The same thing would happen for a base with $K_b$ greater than the $OH^-$.
If the reaction completely occurs, the compound will no longer exist in water.
