Hard to know what happened or what the surveys say, since I also didn't get through the paywall. I did read up on the initial few articles though. It was successful for the first 150 people, and now this after the experiment was expanded.
I know every non-management person hates management, and every low level manager hates the higher level manager, while the higher level manager thinks people below are not good at their job... but that's not necessarily the reason. I think 4 days work weeks work well in somewhat homogeneous companies, especially ones that are project-based. 50 software developers going from 5 days to 4? No problem, we can organize the work for that. 25000 software developers doing the same thing? Probably also possible. For a telecom though, with people in marketing, customer service, IT, etc., it's a lot more difficult. The work is completely different from group to group, Some are customer-facing, and must be present when customers try reaching them. Some need to respond to emergencies (your marketing / comms folks, or just the technicians). So what, do these people never get 4 day work weeks? If you do that, how are you going to justify it to these teams, when the one next door building the next set top box software is working 4 days? Or does the company just hire more of them, thus paying more?