All of those sites offer investments and need to post that information for the peace of mind of their investors. Primedice does not accept investments and our users rarely store massive balances. Users frequently cash out their winnings, often less than a weeks worth of expected profit is comprised by balances.

That being said PD probably has a larger bankroll than any bitcoin dice site currently, at least from what I've seen. We made a simple mistake which allowed ourselves to have 2400 coins stolen from us via exploit but even at that exact moment we definitely had at least 10-20x the total stored PD balances locked away in cold storage. We've since  quickly re-couped those losses and made improvements to prevent that from happening again.

If we were to ever accept investments which I think is very unlikely, we would absolutely have proof of reserves available for users.