Why have capacitors improved so much over the years?

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

"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.