Here's why:
* Storyline: Sirius Black's involvement in the story becomes more prominent in the later books and movies. The events of Prisoner of Azkaban, where Sirius escapes and seeks Harry's help, are featured in Years 5-7.
* Gameplay Mechanics: The game's mechanics, including character unlock methods, change between Years 1-4 and Years 5-7. The earlier game focuses on simpler gameplay, while the later one introduces more complex elements, including character switching and unique abilities.
To play as Sirius Black, you'll need to play Lego Harry Potter: Years 5-7. He's available as a playable character in that game, alongside other characters from the later books.