The operands of the arithmetic operators need to have the same types. That makes the language easier to define and easier to implement, in part because there are fewer cases to handle, and in part because hardware doesn't generally support mixed-type operations, either. Formally, C++ is defined in terms of a virtual machine, but the capabilities expected of that machine are influenced by those of real-world hardware. Operations dictated by the standard but not supported by hardware (such as your hypothetical signed-unsigned division) would need to be implemented by compiler-writers in library code, and that would affect performance.
Either the signed operand needs to be converted to unsigned, or vice versa. The standard committee picked the former.