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.
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.
Actually the previous way was much easier. For myself being in Australia you had AEST. We have 2 states that do not adhere to daylight savings..NT and QLD, so Aust players know how much time to add or subtract at a quick look to determine how much time they have before servers go down. I'd assume it would be the same for the other timezones that were listed.
Last weeks patch said 3am GMT. When I googled that time for when i was on it said it was 6am GMT so I thought, you beauty patch has been done I can do my dailies. Only to be met 45 mins later with "Server shutting down for maintenance in 15mins"
Probably has more to do with the Australian servers being shut down and everyone being put onto the NA servers.