Because GameFreak decides to Shiny Lock them, preventing from EVERY single Zekrom and EVERY single Reshiram to be shiny, and basically make the other ones look less interesting when you are trading with a friend, or via GTS Negotiations. Honestly, I think GameFreak made it so you won't be wasting your time for a shiny legendary, rather than ACTUALLY playing the game.
Also, go check out this page to learn which Pokemons can not be shiny, unless they are hacked.