One, the person who instigates the schism (usually leftists this past decade) have a tendency to simply be unable to not bring it up no matter how casual the topic of conversation is. You can tell them that you disagree, that it's not worth ruining your relationship over, and that all it just depresses you, but even if they fully agree with you, they've become so involved in their ideology that the only way for them to take your advice to heart is walking on eggshells constantly. Basically, either you cut each other off, or you maintain an empty shell of what used to be your relationship. I think
@GrimProphet made a post in this thread earlier (might have been another thread) about cult behaviour, I think that's the best way to sum it up- there's a frightening amount of people that choose ideology over people they knew for decades.
Two, most people just aren't good at debating, or navigating complicated situations like that. I can't speak for everyone, but I have a strong feeling that 5-10 years ago, most of us here would not ever think that we'd have to seriously debate politics or else we'd lose people we trust. It's hard to strike a good balance of logic and emotion especially when arguing with leftists, and more likely than not you'll end up having to defend your reputation because they'll start questioning why you disagree. It sounds really cucked I know, but already you have to be very careful what you say before you're branded a bigot, so you have to hold back how you really feel, because otherwise you'll be dropped by that person.
I guess overall my point is, when someone in your life changes to such a drastic degree, what can you do about it? You can't convince them unless you're an amazing debater, you have to hide a lot of your views (even ones that don't seem that controversial), and at best you talk to them like they're an acquaintance since they're just not who you thought they were. Your best hope is that they can separate politics from other topics, but with how increasingly politicised a lot of things have become, that's unlikely.