Data center =/= authentication server.

The authentication server is in Japan, devs & manteinance team are all in Japan. End of story.
For them to do different manteinance times, they would need to set up authentication servers on each region and have staff working there, this would allow them to schedule different mant times (but on the other hand, woudn't allow you to make chars on JP or European servers since the authentication server would be different, you wound't see those worlds available).

The cost of setting and building such servers is pretty high, that's why no company does it.
Does Blizzard do different mant times to adjust to Europe/Australia time? Bioware? NCsoft? Nexxon?....no, none of them does because their authentication servers are all in NA and because their main customer base is in the NA....ergo their mant is always an off hour for NA but likely a peak hour for the other regions.
Well SE authentication servers are in Japan, and the customer base of Japan + rest of Asia + Australia + Europe surely outweights the customers in NA, so you plan for a mant time that screws the less ammount of customers.
Sorry NA, but we are a minority this time....just the same Europe & the rest of thw world had to suffer in WoW, SWtor, Aion, Lineage, etc. etc.