The problem is that during the industrial revolution it also took a long time because people caught on that 40 hours per week were more effective. It is really hard to reliably measure performance in the long term. Managers are discouraged from advocating a 40 hour work week since this flies in the face of the prevailing attitude. If they fail, they will almost definitely be fired since 'more work'->'more productivity' is the common sense answer, whether or not it is true. It would not be worth the risk for any individual manager to try this unless the order came from the top. Of course, this is not an argument in favor of the 40 hour week, it just shows that this could just as well be explained by a viral meme as by reasonable decisions.

This is part of the reason why I find it so hard to find any objective information on this.