Why have capacitors improved so much over the years?

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? 

Not just capacitors. Chips, transistors, resistors... almost all electronic components had been improved over the years.
Btw, in terms of capacitors, electrolytic capacitors been improved greatly.

I have not been involved in the engineering of any audio product but If I were, I would insure that that no corners were cut on the electronics. Unfortunately I think capacitors are generally the weak-link especially in Class A amps and any given product is only as good as its weakest link. If there were a way to eliminate capacitors, that would be miraculous.