Because you run into scaling problems with both tyrannical and fortified the higher you go. It also gets pretty boring and would get really difficult if at a point you reach a fourth affix.
They already mentioned that they want 15 to be the pillar of difficulties between tiers. In order to do that, eventually they have to re-tune the difficulty of M+ such that lower ones are harder, yet drop better gear to compensate. 7.2 is slated to do exactly that and is suppose to be a reset button to re-evaluate dungeons.
They are doing the same thing to heroic and mythic dungeons, not just mythic. In those regards it wouldn't hurt new players at all, as the rewards at the bottom and entry levels are getting shifted up as well. Interestingly if they didn't decide to do this, the exact opposite problem would exist. New players would enter the system and would be completely separated by higher players by a large margin. Nobody would be looking to run 2-9, when the norm could potentially become above 10 constantly.
Doing this actually keeps the dungeons relevant throughout the entire expansion, and/or actually serving a purpose. And again, if you actually keep the scaling the same you would have to change how tyrannical and fortified actually work.
I thought for sure the first 3 bosses in Mythic NH gave the same as Heroic Gul'dan, but apparently that's not the case.