To be honest, this question would be better asked in an economics sub, rather than here, particularly because the reason why inflation has remained low in the past 20 years is also super interesting, and we can't get into it here.

Anyway, to try to stay in the rules of this sub, the question of what causes inflation and reductions in inflation has been subject to debate for a very long time.

Very broadly, inflation can be caused by an increase in available money, and a rise in wages caused by low unemployment (all briefly summed up as causes of demand increase), or a decrease in supply, and probably many more factors. So as demand goes up, supply must go up to meet it, or else prices start to rise. Similarly, if demand stays the same, but supply goes down, prices go up to make up for the competition over existing supply. For example, in the 70s, we had two major energy/oil shocks where the supply of oil fell dramatically that massively drove up the price of oil in 1973 and 1979/80. And... well, you can see what happened to inflation immediately thereafter in this chart.

https://www.macrotrends.net/countries/USA/united-states/inflation-rate-cpi

What's interesting about 1990 is that, again if you look at the chart, it's really not a super outlier from the late 60s until 1990. Perhaps another person who knows more about that era could speak on the Nixon Shock and the switch away from the gold standard to fiat currency.

Instead, what's amazing is that after 1990, we managed to sustain low inflation for a long, long time. A really excellent overview of how we did it was done by Harvey Rosenblum from the Federal Reserve Bank of Dallas back in 2000. His write up can be found here: https://www.dallasfed.org/~/media/documents/research/swe/2000/swe0003b.pdf

A brief, three word summary would basically be: globalization and technology. Thanks to being able to take advantage of lower wage workers in Mexico with NAFTA, we drove prices down. Over time, ditto in China and the east. At the same time, we had an explosion of immigration, and in doing so, a ton of existing manufacturing moved to the southwest.

If you'll recall, one of the chief causes of inflation is generally considered to be low unemployment, but that turns out to have a caveat. If the low unemployment is driven by workers willing to take much lower wages or whose wages are sent out of the economy, namely immigrants or visa workers who send money home to families in other countries, then, at least according to Rosenblum, the resulting inflation doesn't seem to occur.

Deregulation meant companies could move around more easily and source materials more cheaply. Financial regulation changes propped up banks and made credit easier and safer to get, which made capital easier to get, which made it easier to make capital investments that could drive down costs.

And of course, at the same time, technology and the internet massively increased productivity while reducing cost. Free trade and easy communication made it possible for businesses to constantly undercut each other. And new efficiencies forced further cuts. It's not merely a labor thing where robots replace workers, either. 1999 and the time thereabouts began the rise of Amazon and online stores, massively undercutting brick an mortar stores both by making products cheaper for consumers and but also making products cheaper for manufacturers. The cut online stores like Amazon takes from sellers is far, far smaller than the same cut at a brick and mortar. (I work in supply chain and have witnessed this with every job I've had.)

Anyway, that's the simplified answer. Highly worth visiting the link and checking out Rosenblum's writeup, as it goes into more depth.