My first guess for the reason was simply because of performance and memory saving reasons, and for the ease of compiler implementation as well (especially for the kind of computers at the time when C was invented). Passing huge arrays "by value" seemed to have a huge impact at the stack, it needs a full array copy operation for each function call, and probably the compiler must be smarter to output the correct assembly code (though the last point is debatable). It would also be more difficult to treat dynamically allocated arrays the same way as statically allocated arrays (from the viewpoint of the language's syntax).
EDIT: after reading some parts from this link, I think the real reason (and the reason why arrays in structs are treated as value types, while sole arrays are not) is the backward compatibility to C's predecessor B. Here is the cite from Dennis Ritchie:
[...} The solution constituted the crucial jump in the evolutionary chain
between typeless BCPL and typed C. It eliminated the materialization
of the pointer in storage, and instead caused the creation of the
pointer when the array name is mentioned in an expression. The rule,
which survives in today's C, is that values of array type are
converted, when they appear in expressions, into pointers to the first
of the objects making up the array.
This invention enabled most existing B code to continue to work,
despite the underlying shift in the language's semantics. [..]