I guess I've always kind of wondered why inflation hasn't risen before this crisis, given low interest rates, booming economy, access to credit, etc. So maybe I'm just expecting this to happen at some point, and the impending massive stimulus makes it seem like it's going to be a doozy. But who knows. And probably a problem for future world.
Lots of really smart people have been wondering this for years.
The four most prominent answers - which are not exclusive of one another - are inequality, slow technological growth, a population that is both aging and not growing much, and global trade.
Inequality cuts against inflation because the savers have the money, not the spenders. Saving slows the speed of money, effectively lowering the supply.
Technology hasn't enabled the average worker to become more productive at the same rate as the mid-20th century. The interweb increases productivity massively for a small slice of society. A truck, or a tractor, or a conveyor belt impacted a greater percentage of the population with greater increases in productivity than anything created in the last couple decades. That slowing increase in productivity leads to slower wage growth, which leads to falling demand.
A population that is aging and stable or shrinking is less productive than a young and growing workforce. That lower productivity leads to lower per capita wages, and lower demand.
Last, the globalization of supply chains have been a massively deflationary force because they decrease the costs of goods to consumers. Big box retail stores are deflationary ground zero.
Those are the four that I think most mainstream folks point to as reasons we don't see inflation.
There's also the whole Modern Monetary Theory school now out there arguing that the dollar can never really inflate regardless of supply since it serves as the world's reserve currency. I don't know that I've ever fully understood their logic.