Why have capacitors improved so much over the years?

Since you seem to be interested, one of the best books you can buy is an edition of Art of Electronics by Horowitz and Hill. First edition 1980, third edition just a few years ago.

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.)

@millercarbon - that link has been posted in this thread already.  The original question was actually “why” though…