To avoid any legal liability. Stating a specific reason would open them to possible "breach of contract" depending on whether the act(s) were significant enough or justifiable, based on the contract terms. Just say nothing, part amicably, everyone moves on without drama.
With that said, they probably weren't lying. Most likely, months after ponying up $10 million to a sophisticated international hacking group, Caesars Entertainment probably doesn't want to invite some of the world's best hackers to stay and meet at its flagship resort.