Because there’s no preselected default subscriptions like with Reddit. If it defaulted to subscribed when you make an account, then the first thing you would see is nothing. By defaulting to all, it allows you new users to see all that’s available, and once you find a decent amount of interesting communities and subscribe to them, then it makes sense for you to manually change your default to subscribed at your discretion.
Because there’s no preselected default subscriptions like with Reddit. If it defaulted to subscribed when you make an account, then the first thing you would see is nothing. By defaulting to all, it allows you new users to see all that’s available, and once you find a decent amount of interesting communities and subscribe to them, then it makes sense for you to manually change your default to subscribed at your discretion.