Why have capacitors improved so much over the years?

Tech is nothing more than a physical description of the ways they are different. Why they got better is audiophiles obsessed with and driven to achieve exceptional sound quality. How  they got better was by trying out and evaluating different ideas by listening.

good question, like OP, I would like to know what ... is better and why about capacitors specifically.

Switching from electrolytic capacitors to film caps has brought the focus on the materials used in the construction of capacitors. Electrolytics can sound poor because the insulator is actually an oxidized plate. Basically a capacitor is two plates separated by an insulator. Film caps allow that form of construction. Using high purity metal foils and Teflon insulators without the need to use a paste to oxidize a plate improves the transmission of a purer music signal.

@noromance, thanks. 

I am not sure the best capacitors today vs 15-20 years ago are in fact better. It seems to me there is now a market for high grade fancy capacitors in the DIY hifi community, and so there are some smart companies who are riding that wave and marketing to that audience.

Dueland, for instance. It would be interesting to hear from people who have been in the high end hifi manufacturing business for a long time, hear their commentary on this. There seem to be some others on this forum who will do modifications to some gear for a fee, using these fancy branded caps and other components. Things like salt used to just be salt. Now there is Himalayan salt, Moldon salt, etc etc. It is still just salt, and before there were better quality and worse quality salt for cooking, the marketing business just had not latched on and did their thing.