After receiving feedback on the forums about the limited time players get to prepare for PTR testing, Blizzard responded giving their thought process on announcing and testing new bosses each week.
Lately, Blizzard has only been giving a day or so for players to prepare for raid testing, which limits how many people can get on during the exact testing period.
Each boss only gets 1 hour of testing each time, which doesn't give players much time to organize on the fly and log on ready.
Times are also Friendly for NA as that is where the main Blizzard development team is so it is a late-night for EU and early morning for OCE players, meaning they possibly can't get on with only a day's notice.
Blizzard Detail The Release Schedule Behind Testing Raids On The PTR
The week prior to a test, has Devs confirm the bosses they wish to test and aim to have them ready by the end of the week.
The team tries to eliminate all bugs they can to ensure players are able to participate in the test without any game-breaking bugs happening, which is why they have the internal QA team run through it first before it is pushed to the PTR.
After the QA team is confident it is ready for players to test, they will announce that it is ready and upload the new update to the PTR.
Blizzard knows what bosses they wish to test each week, but hold off announcing anything until they know for certain that they can deliver on the test.
Because the nature of PTR and internal testing can always find extra issues, they want to minimize the chance of having to cancel a test because they announced something too soon.
While it may not be the best timing for players, this system ensures a more stable PTR experience, which then reflects the full release later on.