I'm not sure I get you - "not 0" is a more predicable value than a certain number, isn't it?
Although this does open interesting cases where if you read a bool from one FFI interface and write to another it may have an unexpected value (ex 2). But I still think it is useful for the in-language conversions for example Boolean to LongBool and the True constant to have predictable values.
(Tangentially, VB traditionally used -1 for true. VB.NET uses the same 0 or 1 internal representation for a bool as C# but if you convert a bool to a number in VB.NET it comes out as -1.)
The main reason for this particular arrangement is that, so long as you can rely on truth being represented as -1 - i.e. all bits set - bitwise operators double as logical ones. Thus BASIC would have NOT, AND, OR, XOR, IMP, EQV all operating bitwise but mostly used for Booleans in practice (it misses short-circuiting, but languages of that era rarely defaulted to it).