There are 2 big factors at play:
1. Margins. Manufacturers make huge margins on expensive vehicles and very slim margins on cheap vehicles. The numbers differ, but I think even in the lead up to the 2008 crisis automakers had to sell 5-10 "econobox" cars to make the profit they made on one luxury car, SUV, or truck.
2. Normalization of debt. For many Americans, having a monthly car payment in perpetuity is considered acceptable. Car loans have their place and can be used responsibly, but due to marketing, sales tactics, and cultural sensibilities what often ends up happening is that people start from a monthly dollar amount and then work forwards to buy the most expensive vehicle they can, even if it means taking the loan term out to 72 or 84 months. It's also very normal for people to never pay off their car, instead trading in the vehicle after 3-5 years and rolling equity in the loan over to their next car. Obviously, this consumer habit is great for dealers, manufacturers, creditors and buyers of consumer debt, as well as the US Government and investors -- it's just not ideal for the consumers themselves if they're trying to preserve wealth and build savings.
These two factors create an environment increasingly hostile to the cheap entry level car. Consumer demand is low since most don't spend responsibly, and automakers don't really want to make or sell them because the margins are so slim.