There's a whole bunch of reasons feeding into the shortage:
Demand is at tenfold levels for normal years as a result of COVID and civil unrest; it's also an election year which feeds demand. Around four million new gun owners are also feeding into this.
The major four manufacturers produce about 8 Billion rounds a year for the US consumer market. Production has been disrupted by COVID and requirements for social distancing in facilities.
There is a shortage of raw materials. Even though mining was designated an essential business, production is down so brass, lead, copper etc are more expensive and in shorter supply.
Primers are still pretty much hand made and it is not a short process to train people to do it so this is a big issue.
Trucking is also down about 5% so moving material around the country is also feeding the problem.
Manufacturers have ramped up shifts where possible but don't want to invest into new production lines for what is potentially a short term problem. There are really only two makers of large scale ammunition production equipment and it is not cheap.
This situation really is a perfect storm and is unprecedented. On the plus side, if Trump wins, we should expect normality to return sometime around summer of 2021. If Biden wins it could be a lot longer, if it ever does return.
There's a very good YouTube video which goes through this in a lot more detail but I can't find the darn thing right now. Will try find it and post.
EDIT - Found it! This is long but well worth a watch: