Because it's not a "restriction", LUA literally cannot modify texture files of the game that are used to be stretched on the game objects. LUA is restricted to your UI, game objects are part of the game world, not part of your UI. It's like trying to use your remote to FF world around you
- - - Updated - - -
Because they'll have to remake the game from a scratch. They made LUA to work only with users UI, it can't interfere in game objects, so, to make it work, they have to scrap current model, make up a new one, split UI into two parts, one being "free to modify via LUA", other - just like game world - rendered in other way without user access.
- - - Updated - - -
Remember when people used to draw guides in jpg format? Yeah, the same but in a game world, using players as reference point to draw objects. You just spread everyone out, use an addon to draw circles, pull the boss - boom, circles are there are not going to move anywhere since you are not allowed to retexture in combat