I'm not the least bit surprised that the number of people who never read books has gone up since 1978. There's a lot more out there to distract you. In those days you had 3 black and white channels on TV and portable music was one of these things:
(For the record, while I am too young for 8-tracks my dad still had his collection and we had a 8-track stereo in the garage when I was growing up. When I was a kid my portable music was an AM/FM radio shaped like Super Mario...I was too poor to have a proper Walkman.) Back to books! The point is, people were so bored they didn't have any choice to but read. Now we have Netflix and Youtube and cable packages with 500 channels and the ability to watch TV shows from other countries when we get bored of our own. In this day and age we can and do spend hours a day watching cat videos on the internet.
It's amazing any of us read.
I was an introverted child. My mother says I was shy, but the truth is, I thought people were boring and tiresome. Books were better. Once I really got the hang of this reading thing I stopped talking and I was forced by many an adult to put down the book and go outside and play with the other kids. In the summer between the 4th and 5th grade I read 113 books (I kept a list). As a teen I borrowed the 2nd Harry Potter from a teacher in 2nd period and returned it to him, finished, at the end of 8th period. My record was 3 books in one 8 hour school day. (No, I didn't pay much attention in class--how did you guess?)
But then came the internet. My family got the internet early (especially for a bunch of poor people, but my mom liked credit cards) in about 1996. I started using heavily in 1997 when I discovered Buffy fanfic. I was still reading, but fewer books were being consumed.
Then streaming video and downloading became a thing. It was easier to watch a story than to read one, and cheaper too (books are expensive to people who are not accustomed to $6 coffees every morning). Also, the more I write, the more my internal editor starts to criticize the books I do pick up (guys, I'm secretly an asshole). What it really boils down to is, it's easier to do something else than read. Part of the reason I read so much as a kid was because I was stuck at school for 8 hours a day with people I hated. In the summers an obnoxious woman with horrible children baby-sat me and my siblings, and the best way to get through every terrible day was to hide in another world.
I can see why mothers are big readers. A surprising number of fast-food employees read--a friend of mine works at Taco Bell and the women who work there pass around books like they're joints. We read to escape something unbearable, and when things get a little more bearable (when you like the people you are around and are not stuck in a desk all day long) you find other things to do other than read. Books are escapes, and if you don't need to escape...you just don't read as much.
Now, at 30, I've dropped from the 3% who read over 50 books a year to the low end of the 26% who read 11-50. I probably read around 20 books a year, and as an author that is just terrible. But other things keep getting in the way.
I read when I'm depressed. I read when my fiance is out of town and I'm lonely. I read when I'm on the bus and dont feel like writing. I read when I was at work and I had writers block. I read in line at the grocery store (I get line rage if I don't keep busy--it's not pretty). But other than that...I never seem to find the time.