TL;DR: Because it would mean running away from discussion rather than embracing it, thus making your side eventually lose.
As someone who sincerely believes T_D is full of conspiracy nuts that are really easy to refute, it would be a shame to leave their community to boost their upvot/downvote ratio and increase the perceived credibility of their posts.
This doesn't apply only to T_D. If you care about changing the public opinion and you're not afraid of offenses or dissenting opinions, please leave any controversial subs unfiltered, and downvote stuff you don't like when you happen to come across it in /all.
Right now, that's exactly what the trumpeteers are doing, and it seems to be working. They are actively downvoting posts from communities they don't agree with. I'm not saying we should vote brigade like they do, I'm just saying that if something happens to get to /all and you don't agree with it, make your voice heard and downvote that thing. Don't just ignore it. They can censor discussion and ban you from posting, but they can't take away your right to downvote something they're shoving in your face.
By running away, you are ensuring your loss in any discussion. It's the type of attitude that gave Trump the credibility to get elected in the first place. It's time to grow some thicker skin and change that behavior.