Because they work.
Source: Being a political strategist/local politician in a Western democracy. Have had a lot of fun with highly effective negative campaigning.
it works.
But it works most effectively when you associate the positive elements of your campaign with the negative elements of what the opposition is (supposedly or actually doing). E.g. if you're campaigning on the opposing party squandering the municipal authority's money through reckless spending, you emphasise that you are committed to bringing fiscal discipline to the partyauthority. Amateurs will just bash the opposition; pros will bash the opposition and promote themselves at the same time.