Blizzard never says exactly why, they use the exact language from the ToS which is itself vague. Any mechanic that allows you to defeat mobs or bosses by using ledges and or pathing is exploitive. Bots and boosters used these exploits to defeat the dungeons not as intended. If you outgear a dungeon and aoe down a bunch of mobs, that’s not exploitive. If you jump on a ledge to reset or force the mobs to path differently, that’s exploitive. Vanilla was v1 of a game that was more polished in v2, tbc.
Pirate server players found every exploit in all the dungeons to defeat mechanics as a solo player. Blizzard doesn’t seem interested or capable of physically patching the terrain. They are leaving them in, but limiting how often you can do it with the caps. Unfortunately the pirate server players who made you tube videos on how to defeat mechanics with exploits, caused a cap on all dungeons, because blizz isn’t going to patch the terrain.