It's adversarial, because it's good strategy for winning elections. In a two-party system, there's no need to compromise. All you need to do is to beat one person (generally speaking). The worst way to do that is to agree on anything with the other person. You want as many wedge issues as you can, if your constituents are in favor of your position. If they're not, you attack the other person's character. Preferably you do both.

It's been this way for a very long time, and it's only getting worse.