Opposition to abortion in the USA was originally largely driven by Catholics. Which, incidentally, meant that most American Protestants didn’t especially object to it, including the majority of the Justices who issued the Roe decision. Eventually, for a number of reasons, many other Christian denominations, especially the poorer extra-urban ones, came to agree with the Catholics that maybe killing unborn babies really is murdering an innocent human life.

Meanwhile, my understanding is that Catholics haven’t been a major political power in the UK since the Jesuits were expelled.

As for why it’s a “political thing” in the USA? Ironically, that’s largely because of Roe. Roe removed the abortion debate from the elected branches so that instead of working something out in a healthy democratic fashion, we got a naked exercise of judicial fiat.

This next bit is personal. I hope people will read it as a desire to satisfy curiosity rather than an attempt to debate a subject that, frankly, has adequate firebrands. For Catholics and those others who agree with our moral reasoning[1], abortion, the killing of smaller helpless innocent human beings, is an intolerable enormity. It’s completely impossible for us to knowingly condone it without falling from a state of grace. And doing nothing is condoning.

[1]. Premise 1: killing innocent human persons is wrong. Premise 2: pre-born babies are innocent human beings. Conclusion: killing pre-born babies is wrong.

The argument is simple enough to follow. Note that, due to the structure of the argument, adding additional premises like “the father is a rapist” doesn’t change the moral calculus unless we add an additional premise like “at least some offspring of rapists are not innocent merely due to their paternity.” Since the argument is structurally sound, we must challenge the premises. Challenging 1) is possible, but permitting killing innocent human beings opens a big door to all manner of atrocities. Challenging 2) is the more seemingly reasonable approach. Catholics believe that the human person comes into existence at fertilization, when the diploid human organism begins. One can reasonably use some other event like heartbeat, or some more vague concept like viability, and claim the human person doesn’t begin until then. I’ve yet to hear a convincing argument though. I personally lean toward the fertilization point, because when getting the call wrong means murdering an innocent person I prefer to err on the side of caution.

The most popular arguments against protecting an innocent and dependent human person though are completely unprincipled and not properly even arguments at all. Rather, they they reduce to some kind of hedonic assertion on the level of “I’ll do what I want!”

Edit: Please don't reply trying to goad me into some low-brow argument. I am not interested in poorly thought out analogies or any other simulated ratiocination. For the purposes of this discussion I am only interested in the ethics. So by all means please provide clear moral reasoning about why it's OK to kill innocent human persons sometimes but not others. Please explain why it's OK to make it a legal requirement to care for some vitally dependent human persons but not others. And please fully think it through and discuss the potentially unintentional consequences of adopting that ethical system.