When Disneyland opened in 1955, Walt Disney decided the park should be dry, which is to say, alcohol-free.
However, Walt Disney died before the next park opened at Walt Disney World and the decision was made that while Disneyland and the Magic Kingdom would remain dry, other new parks would sell alcohol.
When the Magic Kingdom opened at Walt Disney World in 1971, the same rule was implemented.
He wanted to separate his new Disneyland experience from the carnivals and circuses that came before it.