Probably for the same reasons that 1-bit signed int bitfields are UB. I suppose they could have allowed it as you say, but... is there a need?
It does say bitfields of size zero are allowed (that forces padding to the size of the allocation unit), and also, curiously, says that, in C, “whether int bit-fields that are not explicitly signed or unsigned are signed or unsigned is implementation-defined.”
Prior to C23 there was no guarantee of two's complement, so it was possible that such a field could only hold the value 0.