[mullr]

Because in a statically typed language, you want the type of a symbol to represent the set of values it can be bound to. We often have the set {something that passes a truth test, something that fails a truth test} in our programs, so a boolean type is a nice way to represent it.

Now, this is not the same thing defining what exactly the truth test is. It's perfectly reasonable and arguably a good design to have both a narrow boolean type and a wide definition of truthiness.