Originally Posted by
Eurhetemec
Pretty sure it's to stop people spamming Mythic (non plus) dungeons over and over hoping for Warforged/Titanforged gear.
Sure, you can do Mythic+ indefinitely, but that takes actual effort as you have to find people, and then have to stop using keystones when they get depleted or things just get too hard. You may get better loot, but it's harder to get, where normal Mythics could easily get pretty mindless.
Personally I think that's a bit of a silly reason but I'm pretty sure that's the reason.