Abortion is what's called a Wedge Issue[1]. It is so because the public opinion on the US is divided roughtly 50% for it and 50% against it.
On top of that, the US presidential election is a First-past-the-post[2] system. So if I manage to get 52% of the votes and you only get 48%, I win everything, you lose everything. You can probably imagine where this is going: Instead of convincing 51% of the people I only need to convince 3% of the indecisive, and I win.
Finally, the US is a very polarized country. The "other" is always bad, "we" are always good. So the wedge issues tend to "align". If you and I agree on abortion, we will probably also agree in most of the other wedge issues.
All of these factors together result in that both Democrats and Republicans are forced to "optimize", so their campaigns all revolve around the same wedge issues. They must, if they want to win.
If you ask me, the least complex way to get the country out of this rut would be changing the voting system to something other First Past The Post.
Unfortunately, the people who are in a position to make such a change are the least motivated to make it.
[1] https://en.wikipedia.org/wiki/Wedge_issue
[2] https://en.wikipedia.org/wiki/First-past-the-post_voting