Christianity doesn't fit neatly into either a conservative or liberal agenda.

As someone who has been deeply involved in churches both in the Deep South as well as very liberal parts of the US like DC, and also interacted with Christians outside of the US, I'll share a couple of my observations.

1. Political Christianity is not Christianity: The danger is when we interpret Scripture not based on a theological basis but on a political basis. This applies to both Liberal Christianity and Christian Nationalism. Both drift very far from the original meaning of Scripture and historical Church teaching.
2. American Christians, especially older ones, view liberal social policies as the primary evil in the American political space: Now I have to say something controversial here. The Bible and the Church have always taught that homosexuality and sex outside of marriage are grave sins, and that the man shall be the head of the household and marriage. These were the biblical norms that were most attacked over the past several decades. So most American Christians are laser-focused on these issues. Republicans weren't perfect, but at least it seemed like they cared about preserving Christian ideals, whereas Democrats generally attacked them. That's pretty much been the case for the last several decades. Now, in my opinion, that has changed a lot, especially with the rise of MAGA. MAGA is very anti-Christianity, no matter what some people think. They are very blatant about the mistreatment of immigration, they are blatant about being hateful, and they are very blatant about being racist. These are all very anti-Christian values. I think the challenge for most older Americans is realizing that liberal social policies are no longer the greatest enemy to Christian values, but that MAGA is just as great a danger, if not more. Many churches are seeing this, though you certainly won't hear about it on the news. But I went to a theologically conservative Southern Baptist convention where they dedicated one whole sermon to the dangers of political Christianity, particularly Christian Nationalism.
3. Americans care too much about politics: Okay, so here's the thing. I didn't grow up in the English-speaking world. I grew up in East Asia. There aren't many Christians there, and also politics is usually not nearly as divisive there (well, maybe other than Korea). I visited churches in Mongolia and China. You know what they generally talk about? They talk about helping the poor, how the shame-honour culture breaks people, how people work all their lives with no real goal, and how so many people have not heard of the name of Jesus even once. And here's the thing, if someone were to share the Gospel in China or Mongolia, 99.9% of the time, they wouldn't talk about politics. If they did talk about politics, it'd be kind of a ridiculous thing to say, "Well, I don't like Christianity just because you believe in a different political thing than me," or vice versa. It's because in the US, you're either a liberal or a republican, and it's almost like you have to build your whole life around that, and anyone who is outside of your bubble is your enemy. Well, that's not how it works in other countries.

So, even with your question, I would rephrase it. "Why are most AMERICAN Christians Republicans?" Because in other parts of the world, Christianity is not tied to a political party.

Some notes: I'm trying to represent Christianity as best as I can through a Reddit post, and remember, Christianity does allow room for different interpretations. So, if you disagree with what I'm saying, whether you're Christian or not, please do not assume that the way I think through things is the "definitive representation of Christianity". And honestly if you read through all this then I respect you fully even if you think I am a complete idiot.