In the current voting system, and in fact in most voting systems, negative politics and attack ads are effective strategies. You can be as nasty and divisive as you like and attack your opponents. You don’t need to care about alienating the people who support your opponents: as long as your core supporters vote for you and there are more than two candidates, you can win even if most voters heartily dislike you.
A ranked ballot, on the other hand, gives an advantage to candidates who are conciliatory. In order to win when there are more than two candidates, you need to get the second-choice votes of people who support your opponents. You can’t do that if you attack the candidate who is their first choice, and if you call their ideas foolish or dangerous. To win, you must reach out to those who like your opponents and find what you have in common with them. Being nice to opponents and accepting some of their proposals earns you the second vote, and being polarizing does not.