Why have capacitors improved so much over the years?

"Why" was answered in the third post.

I have no idea what I’m even looking at with those air caps!

No one does. Nor does it matter. What matters is how they perform. As determined by listening. Which is what I said in the beginning.

If the real question is not why but which this is how you go about finding your caps- study this. Everything you need to know. http://www.humblehomemadehifi.com/Cap.html This was my main resource for upgrading my crossovers. (Thank you, Rick.)

Tech is nothing more than a physical description of the ways they are different. Why they got better is audiophiles obsessed with and driven to achieve exceptional sound quality. How  they got better was by trying out and evaluating different ideas by listening.