The National Pokédex has been a feature in the games since 2002. In this game, like in FireRed & LeafGreen, it is required to continue the game's story and required to get over 20 of the new Pokémon:

Obtaining the National Pokédex

Unlike in Pokémon Emerald when you had to beat the Elite Four to get the National Pokédex or FireRed/LeafGreen which required you to get 60 Pokémon; the Sinnoh National Dex requires you to do two things and two things only. Defeat the Elite Four and see the first 150 Pokémon in the Sinnoh Pokédex (This excludes Manaphy), not actually capture them. This feat is very easy to do, and by the time you have beaten the Elite Four, you should have at least seen over 135 of the Pokémon in the Sinnoh Dex.
Some of you may ask; "How do we see Dialga/Palkia? I have Pearl/Diamond and they aren't obtainable in it." This is easily remedied. After beating the Elite Four, just go to Celestic Town and go to the top house. In there, talk to the Old Woman and she will tell you about the other legendary and show you a picture. This adds it to your Pokédex

Now you have seen all 150 Pokémon, go back to Sandgem town and speak with Professor Rowan. He will analyse your Pokédex and note that you have filled it. At this point, Kanto's Professor Oak barges in and notes this. He gives you the National Pokédex and tells you about all of the things you can do now, including accessing new parts of Sinnoh and transferring your old Pokémon over through Pal Park