So one factor here is the actually computers: 50 years ago, store clerk was a medium prestige job, that had medium pay and growth prospects- there are plenty of stories of people who started a clerk and ended up in a senior position. But that was because clerks had to know the prices and goods: a major risk to the store owner was customers swapping price stickers, so you needed a clerk who could spot the difference between the expensive onion and the cheap one and knew what the price should be and these clerks had to stay in one place long enough to know the prices.

Then along came the laser/UPC/computer system and deskilled the job. No longer did you need to know the difference, the computer printed up a natural language description of the item. This allowed stores to get much larger, because no one could know all the prices in a large store (early department stores handled the size by making you pay for items in each department, using store balance accounts to not create too much friction). This fundamentally reshaped the relationship between capital (who owned the stores and the computer system) and labor. It's actually the exact same alienation of the individual from their labor that Marx was writing about in the 1850's, just for jobs that didn't get the same attention.

Similarly, an in-law worked as a teller at a bank. 50 years ago that job would have more upward mobility because banks managed their risk locally: after many years as a teller and then as a loan officer you could move up to be the local person making the decision on loans. Nowadays large banks have centralized that with fancy computer systems to guide the decision, so there are entire paths of advancement that essentially don't exist anymore because computers spit out the answer. There is only shift and then branch manager, with once again the owners of the computer system using it to de-skill jobs and cut off promotion opportunities. (See, e.g. Better.com, using poorly paid contractors to do all the tedious stuff that can't be automated, and then letting the computer do all the high skill work.)

This is, by the by, one of the reasons that computer programmers continue to get paid well, because we are enabling capital to de-skill and bifurcate jobs better and better.