Beacuse all Pokemon can be Shiny.There are just some Shiny Pokemon that cannot be obtained legally.
And like I said before :
Unobtainable Shiny Pokémon
In most generations there are a few Pokémon that cannot be obtained Shiny without cheating or glitches, due to either being distribution-exclusive without an in-game location or, in Generation V, being programmed so that they cannot be Shiny in the wild.
Unown forms other than I and V