Why have capacitors improved so much over the years?

"Why" was answered in the third post.

Please define "improved".

In terms of sound quality, I do not think much has changed.  30 year old polystyrene caps were excellent, they still are.

If however, you are referring to their reliability, closer tolerances and higher temperature ratings, then yes, the more modern ones have greatly improved relative to their counterparts.

I want to submit perhaps it's equal part capacitors and equal part tweeter performance which has greatly improved.

@cakyol so you think it's all hype?