There's usually not one reason. It's like asking why a person chooses one game console over another.
The "abortion stops a beating heart" argument is obvious. You're alive and have a soul from conception, so it's taking a life to perform an abortion. Whether you agree with that perspective or not (most in this topic won't), that's how many of them view it.
If an individual is of the mindset that what promotes Christianity is good, what promotes or enables secularism or adherence of other faiths is bad might feel that the left is not Christian enough. Or, because factually some leftists are opposed to religion as a whole, or Christianity in particular, they may generalize and view the left collectively as the anti-religious or anti-Christian side.
Then you have Israel.
Then you have the religious authority / vote of confidence approach. The clergy, the church, are the people closest to God. They receive information more directly from God than what we do. So if they say this is the side which represents God's faithful, it must be so.
And then you have personal interpretation and cherry-picking. Saying that you should give your neighbor a coat if you have two and they have none may sound socialistic, as might distributing the fish and loaves of bread to all, but it's not. It's not because I want to make money -- even if I'm told the love of money is the root of all evil. And the quote about the poor always being with us has historically been translated to ignoring the needs of the poor because it kept be helped; poverty will always exist.
The basic control factor. Everyone wants to exert their influence and they feel this is the best side to align with in that regard.
Finally, you have the vote of confidence. If you believe God is your friend, you can convince yourself you're justified in all you do. Leaders have literally justified murder under this premise and it's how bigotry is cultivated in spite of teachings to the contrary.
Oh, and there are at least six or seven reasons I hadn't even thought of.
I know it sounds a lot like I'm vilifying Republicans, but that isn't my intent. If I gave a list of reasons why one demographic or another voted Democrat, it would likely sound dubious on paper. I just wanted to go into some of the specifics the best I could. It's hard to talk about politics without making it sound like someone has an "agenda." Chances are, they do.