Why is the article the used when talking about the death penalty, but isn't used when discussing abortion?
The death penalty is referred to with an article because it is a specific thing—a penalty. It refers to something written in law, rather than all instances of it. There are multiple instances of the death penalty, referred to as death sentences.
In the linked article, another term for the death penalty is capital punishment. This does not use the definite article because punishment is a general idea—a term for the imposition of a penalty. We could, of course, say the punishment but that would refer to a specific instance of it.
Similarly, when talking about abortion, it is the general idea we are talking about—not something written somewhere in law books. There is no specific abortion we are talking about, no specific thing we can refer to as the abortion, unless we talk about an instance of it.
Here's a web page about when to not use the in English. Similar general things that do not need articles include
Whereas, when we refer to a particular instance of these things:
Emphasis and politics. By using the the term "the" in front of a word in a title, it's emphasis that term and also narrows it down to a single topic.
As a reader, which one do you think has the more meaning and impact? I could talk about 'a' death penalty, but when the word 'the' is mentioned in front of a penalty, it becomes a single entity.
Imagine the difference between The Death Penalty and Abortion.
When using the term "the" in front of abortion, it's meaning changes. It has more emphasis and is no longer consider a loose terminology! Both the death penalty and abortion involve deaths, but one type of death is considered "more important" than another death. That's the political half and the reason why "the" is used for the death penalty compared to abortion.
The older practices were "sacrifices" in which youth and newborns were killed in order to appease a God in a ritual. Today's unborn youth (Generation Z) are killed by their parents (Generation X) through abortion by help of medical science. When there were wars, parents were concern of the life of their children, but since there has not been another world war, this care has been replaced with politics which extends past from the Baby Boomber (BB) generation.
It's quiet strange how the human eyes and ears are trained to react to certain words in the English language. There is a reason for it. :)