Unlike public ones, the community group is not searchable and you'd need an invite link or be invited. I'm not sure why they opted for a private one as well. Publics would have better visibility and much easier for people to join. But they could always change it.
It is very possible that it is a close group for the now so that you can keep it controlled and control the people who join. If they leave the group public when the management is not ready to devote full attention to it, the group can be flooded with scammers who may be looking to look for victims from the group chat and that can end up bringing a bad name to the company. I'm very sure that the company knows that a public group is better and later in the future will have the intention to open it up.
They can always set a bot to moderate posting of links and other things that will lead players to being scam. I think main reason for private group is for exclusivity probably they offer some exclusive bonus in there that is only available to certain users that reach the required VIP level.
Honestly, I preferred the private group over public group since it’s clean and moderated properly. An d at the same time less competition on claiming bonuses.
What Queentoshi said is correct private group is the best for now because if the group is open without a proper moderation, scammers will use the opportunity to scam people in the group in the name of the brand and it will affect their reputation.
I have seen some groups using bot to moderate it users but we still see scam complains in those groups and that means using bot is not the best as well. But it helps temporal moderation and also it limit the level of spamming the group.