I believe there is a different reason. For starters, let's not pretend Lily and Noah were even remotely exciting to watch. They very rarely had friends over and the friends they did have over were equally boring. Even if they wanted to remain a monogamous couple which is fine, Noah was a very uneventful performer sexually. Granted I've only seen a handful of sex between them but the sex I did see nearly put me to sleep.
Em and Sid are far from exciting themselves, but they have the added benefit of having two scandals behind them because of Em's cheating which is keeping people glued to them, plus they have a network of other tenants such as Nyusha and Lisa that all know them and visit them.