Market, design
Boats have shoal and deep draft keels because the market (we boaters) want this option depending on our sailing location, needs, and plans.When designing a boat, it is easier to solve both keels at the same time. Likewise, when the ballast/stability is tested by inclining the new boat and measuring recovery forces; solving the weight and shape of the keels at the same time is most effective.Anyway, that's my idea.DavidLady Lillie