This is a great question! We looked at how we were handling our maintenance messaging and decided a couple of things. The number one goal of the messaging was to have someone be able to look at it and quickly discern what was happening and when.
Why have they changed the patch times from showing the time zones and only showing GMT. This is now as annoying as WoW's patches were. In that you come home from work, think you have time to get a couple of hours of playtime in, only to be greeting with..server is shutting down in 15 minutes.
The problem with our old maintenance messaging is that it was very convoluted, just a bunch of information overload. Although I can appreciate how nice it is to have a lot of timezones listed, it creates more opportunities for us to mess up or for someone to read it incorrectly.
We feel that most players should be fairly quickly able to convert the time out of PDT and GMT to discern maintenance times.
I hope that answers your question on why we made the change.