Becuase We All Suck
This is because you don't have the national pokedex.
It wont let you because the new Pokemon cant go onto the older game you can only transfer Pokemon from older games on to black/white
What you have to do is go into the settings, type in the name of the game and then go to add a new code. To get some good codes go to Google and look up action peplay codes for Pokemon platinum. Select the very first website.
You must insert Platinum into the AR, and on the front screen, a code is shown, and it will say "Unknown Game". write down that code, then go to the game list screen, hit "Add New Game", then type in the code. the only downfall to it is that it does not have pre-programmed codes, unlike an AR that has platinum already on it.
you go to your local game shop and buy it.
internet or a gamestop u can buy a book with all the pomkemon in platinum
go to new game and type in Pokemon Platinum for the name and this for the game id : CPUE-D074D1B3
Giratina can be transferred, but it wont be able to go into its origin form in pearl
Go outside and Get Physical Activity
you go and poop on your game. Then you give your game to me.
if you have Mew on a game boy Pokemon game migerate it to platinum and than go to the pal park.
You have to either hack the game or go to an event.