I'm pretty sure I know why... The working environment changed. Bit by bit I moved from an environment with gentleman's agreements and so on towards something in which everything was codified, safetyism became rampant, etc. In the UK over the past few years I went from going into a nice office in a number of beautiful old buildings, getting my stuff done with cameraderie, having lunch together and playing board games,…
That said, I don't have an overly long commute, so I totally get it for those that are able to be more productive due to not spending hours in the car every day.
I notice that when I'm working from home, I'm much more likely to goof off and be less focused. Plus my wife being there is always distracting me with one thing or another. I'm pretty sure I'm quite a bit less productive working from home.