Because piracy isn’t legal. For anything that can run afoul of the law, or bad publicity, or advertisers’ preferences, Reddit admins have to keep the content on a tight leash. Lemmy doesn’t have advertisers to worry about as it’s supported directly by users, and not being a for-profit company makes it somewhat harder for the law to come down on it (and if they do, the community can easily move). Really, it’s a fundamental advantage of federation.