Why have capacitors improved so much over the years?

Vintage Japan Man collectors pay their weight in Gold Pressed Latinum for formed and working 50’s production TCC Metal and Super Metalpack paper in oils.....

Some good answers. Many new cap technologies driven by military applications primarily. New materials and improved manufacturing processes. Electrolytics have gotten smaller, but basically unchanged, still used because they are cheap. Capacity/cost.

Capacitors have improved in certain ways due to designers looking and wanting a different type of sound than previous versions. The change in sound is due to using different materials and construction. I can say that the early capacitors due not sound bad at all if given time to re form in an old component. You have to give some time before you judge an old capacitor especially in an amplifier or speaker because most of the time they are not bad at all they just need time to play and re form. If you do change the capacitor to a new one it can sometimes cause the gear or speaker in question to sound very different, especially bright at first, due to the new cap needing to form as well.

The best are still vacuum dielectric or air dielectric. They’ve been around forever, but they come in small values at high cost. I use these exclusively in my phono / pre.

Film and foil are next best; styrene is pretty neutral, teflon a bit bright, polypropylene a bit dull. Some examples are ’very’ rather than ’a bit’.

Metallized are constructed by depositing metal onto the dielectric. Quality appears to depend on the thickness of the metal and the junction of the metal to the wire leads. They can be almost as good as film and foil, or almost as bad as electrolytic. Mica and teflon are the best.

Ceramics tend to be inferior to metallized, the quality seeming to depend on the dielectric constant of the ceramic, lower being better.

I don’t have strong opinions on paper in oil, but I really don’t like leaks, so I avoid them.

Good luck!  YMMV