I also think that it's because they just don't take that many reservations. Guests with reservations are "guaranteed" a table in the downstairs, restaurant portion of the venue. The main benefits to a reservation are not having to wait in the walk-up line, access to the (full) restaurant menu, and that you can usually choose whether you want to be inside, outside, at the bar, or at the communal table downstairs. Walk-ups aren't always allowed to choose (sometimes they do, but not always). The upstairs seating area (old Cove Bar) is no reservations, so there is a fair amount of walk-up tables, but it doesn't have the full restaurant menu (no larger entrees or the donut dessert).