Why have capacitors improved so much over the years?

First off the capacitors of the past are not all around today.  Think Black Gate capacitors which I feel were the best capacitors back then and today but they are no longer manufactured.  Materials have not really changed but now manufacturers are using different materials then they did back in the day.  Same goes with cables.  Silver, copper, aluminum combinations, twisting, insultation, etc.  Even the cheap Sprague capacitors lasted a long time.

What is different is more of the wire used.  Dueland makes hybrid copper and silver capacitors.  Could not find that back in the day.

So to me it is more of the types of materials being used and yes, some are made better than others - think V-Caps.  They are a really good manufactured product.  As referred to above with parts swappers - you can change the sound by installing larger power supply capacitors think Nichicon and Mundorf.  Anyone of you can do this as there is generally two leads for solder wires onto - just make sure there is enough room for the larger caps.

So what has changed to me is the materials used, etc.  BUT some of these caps can elevate you systems sound considerably.  V-Caps especially.  

For resistors - Audio Note now only offers non-magnetic resistors.  A step up from their now discontinued resistors.  Plus they also are now manufacturing a Silver resistor.  Expensive but worth it.

Happy Listening.

Richard Marsh published an article in Audio Magazine in 1980 that detailed the range of parasitic loss factors inherent in capacitors. Dialectric Absorption and Dissipation Factor in films, and Equivalent Series Resistance in electrolytics. He compared mylar, polystyrene and polypropylene and teflon films and found each had a distinct effect on how a circuit sounds. The race was truly on after that. Prior to this publication only Mark Levinson was featuring Wima caps, but soon everyone was going boutique. I think military, aerospace, and medical equipment advances created the market and audio was an accidental beneficiary at first. 

Around 1978 or 79 I had a gentleman answer a for sale ad I had posted for an Audio Research preamp. He made an unusual request of me. He professed to be the owner of a company that was prototyping a preamp of his design. He asked if it would be possible to compare his prototype to my SP6. Who would turn that idea down? He arrived with his unit that was basically just his boards and electronics. A true early stage build. I didn't expect much. I was wrong. We spent a couple of hours using my system and comparing the two preamps to one another. I only remember that he was convinced his sounded more musical and that he was concerned that his choice of capacitors was in need of change. He explained to me what he was listening for concerning the caps. At the time I thought it was "out there" that he would be sitting in his lab and listening to various caps. He must have done something right because his company is still in business and still selling a highly regarded preamp. 

For the interested:
Picking Capacitors - Walter G. Jung and Richard Marsh (reliablecapacitors.com)
No affiliation.

Everything has improved over the years.
Defects introduce colorations.
Some prefer the colorations.