Because encryption doesn’t work for rooms over 50 people, so any room over that size is public by default.
By public you mean non-encrypted? How does that work? When you create a room, you default to encryption, and there is only one participant (the room creator). And you cannot turn off encryption, so what then happens when you get 51 participants?
Also existing non-encrypted rooms are never automatically switched to encryption, so the switch must be explicit. Does it refuse to do it if there are more than 50 participants?
I've never heard of this limit nor was I able to find info about it (so a link would be great), but there could some factor that increases problems as the number of people increases.. Perhaps 50 is some practical suggestion for the maximum number of people to have in encrypted sessions?