If it's a new casino and they have a huge bankroll and want to impress and invite new players to play on their platform because they have proof that they can play, it's a good show-off to make their bankroll public.
But if it's a big and well-established casino like 1Win they don't need to do that; they just have to protect their reputation. Big casinos have proven that they can pay millions in winnings and can do massive promotions and giveaways.