The gaming industry is a multi-billion-dollar market that relies on the sale of games, DLCs, and other digital content. When gamers download cracked files, they are essentially depriving the game developers and publishers of revenue.
Assassin's Creed Brotherhood is an especially popular target because of its age. Hackers know that security patches for older games are non-existent, and antivirus software is less likely to flag cracks for "legacy" titles.