OK you are questioning the explanatory power, not whether times have changed in the short run. Incidentally in terms of how times have changed in the long run, it is worth remembering that the 19th century may have had less lesiure than we do, but the long term trend (at least from late 19th century forward) was for a drop in the work day. So when paid work start increasing again in the mid 70s that reversed a major long term trend; you might expect some sort of consequences. Changes in sign might account for things that absolute quantity does not.
However you also raise an interesting question: where did USAins in the 19th and early 20th century find the time to do stuff like build a successful labor movement. Where do people in absolute poverty in poor nations who often live lives of absolutely grinding drudgery get the time. I used to be able to do without sleep when I was 20 for long periods of time but cannot do that today. There is no way I could be an activist on top of working a 60 hour week, I mean I don't believe I could physically do it. I would absolutely worthless to any movement I worked in if I was working 60 hours per week at a paid job. I do remember that movements had a lot of full time leaders who were paid in money or in kind (the famous "pie card" in the old CP) . I do know full time activists who live on in-kind donations. People let them couch surf, donate meals, people donate money for their car repairs. There must of have been a lot of that: people who could not give time and who had no money still managed to make sacrifices to help keep the people who did give time going. Plus more prosperous workers and middle and upper class supporters gave cash of course.
Also, I dimly seem to remember that 19th century farming was extremely seasonal in terms of labo/leisure. During winter there was not a lot you could do. Some repairs. and wood chopping and so forth, but no planting,weeding or harvesting or crop processing. So there were periods when you had tons of leisure, were maybe working a 20 hour week. But maybe this is a myth.