That’s part of it. The fuel economy is not better with a manual nowadays. (Or marginally so.) And many never learn how to drive one, because even the economy cars sold here usually have an auto except on the very bargain-basement trims.
My first car was a manual and every car I owned through the 90’s was, too, even higher end Japanese sport sedans. But, with population growth here it just got to be too much of a pain to row through the gears in stop and go traffic. That, and an old high school cross country injury to my left knee started bothering me with the clutch. I still have one manual transmission car, but I only drive it out into the country where I can enjoy it. I hate seeing people blabbing on their cells while driving, they drive worse than drunks with the weaving, crossing multiple lanes to turn and running red lights. So, I try not to pick up if I’m driving and usually don’t unless it’s something that seems very important. Even then I’ll find a place to turn off and stop if in heavy traffic. Out in the country, I’ll keep going but there’s not as many potential distractions on top of the cell phone itself so it feels OK to me.
There’s another issue - commutes have been getting longer, and ‘extreme commutes’ are on the rise. https://www.washingtonpost.com/news/wonk/wp/2017/02/22/the-american-commute-is-worse-today-than-its-ever-been/
If you’re going to have to spend three hours in your vehicle every day, do you really want to spend that three hours in the vehicular equivalent of a jail’s visiting area or holding cell? Or do you want it to be a comfortable place so the three hours aren’t so unpleasant?