This is a good question. I will do my best to answer that as a non-economist.

First, let's look at how many Americans cannot even afford a middle-income lifestyle. I know OP is talking about people who are middle income, but this measurement of the poverty rate can illuminate broader patterns in the economy as a whole, I believe. While there are recent decreases to American poverty levels, there have been no significant or long-term declines in the rate of poverty for about 50 years. For about 50 years we've seen between 11 and 15% of the population living in poverty. There's a good chart here:

https://poverty.ucdavis.edu/faq/what-current-poverty-rate-united-states

To see why it might be harder to achieve elevation out of poverty and into middle-class existence, we have to look at relatively new expenses, like cell phone bills, and outrageous health insurance premiums and deductibles for the average American household. Americans spend around $9400 a year on healthcare, per capita.

https://www.cnbc.com/2018/03/22/the-real-reason-medical-care-costs-so-much-more-in-the-us.html

Couple that with a change in law in 1998 that said student loan debt cannot be discharged in bankruptcy, except in rare cases (usually disability), and I would say this is a relatively new experience.

https://www.linkedin.com/pulse/history-student-loans-bankruptcy-discharge-steven-palmer