Sirius replied, "The Fidelius fails when the words of the Secret are no longer true. I’m the only person who remembers the former Secret for this house, but that Secret referenced the Order of the Phoenix. If Dumbledore would’ve disbanded the Order, this house would’ve become visible. This way, if next month I buy a house in Crawley or Little Whinging and go live there, the Secret still is true and the Fidelius here on Grimmauld Place remains."