Thye found (in the 1930s) that the reduced work week paid for itself in reduced workplace accidents. However, after WW2 companies moved to employer-paid accident insurance instead of paying out-of-pocket workman's comp, and as a result, the cost savings would be socialized anyway. This tilted the equation back toward an 8-hour day, at least from management's perspective, though employees remained staunchly opposed to the longer day for a generation.
What ultimately killed it was a status thing: by the 70s and 80s, employees on the 30-hour work week felt like they were "slackers", and didn't have the same social status as those who worked harder.
On January 5, 1914, the Ford Motor Company took the radical step of doubling pay to $5 a day and cut shifts from nine hours to eight, moves that were not popular with rival companies, although seeing the increase in Ford's productivity, and a significant increase in profit margin (from $30 million to $60 million in two years), most soon followed suit.[10][11][12][13]
If true - that's really interesting. I wonder what the other outcomes / configurations were.