If all a candidate has to do to get elected is be less evil than the other, then there’s no incentive to be good; instead candidates are free to be as evil as they want up to the limit of the opposition. If a candidate had to be less evil than the person they’re succeeding, sure, we’d get less evil over time. But that’s not the case, they’re compared to their opponents not their predecessors. Every time we elect someone shitty we show them they don’t have to be any better than that to get elected.
If all a candidate has to do to get elected is be less evil than the other, then there’s no incentive to be good; instead candidates are free to be as evil as they want up to the limit of the opposition. If a candidate had to be less evil than the person they’re succeeding, sure, we’d get less evil over time. But that’s not the case, they’re compared to their opponents not their predecessors. Every time we elect someone shitty we show them they don’t have to be any better than that to get elected.