It's because of the First Past The Post voting system that is used in the US, which means winner takes all.

If you want to understand why the US seems so uniquely politically divided, then you will understand once you understand how FPTP voting works and how it inevitably leads to binary politics. Most countries have a voting system that leads to broader representation. US politics will always be black and white and divided so long as FPTP voting is used.

There's a great video by cpg grey explaining FPTP: https://youtu.be/s7tWHJfhiyo