NOR is quite useful in combinatorial logic functions, but has few obvious uses in program code where there are typically other complete implementations of the sort of operations of which NOR could be a circuit-level building block.
Bitwise operations in program code tend to be more in the nature of bit setting, masking, or XOR toggling, complementing, etc.
Where NOR would be needed in a control application, it's likely to be a slow enough requirement that an additional inversion operation would not be an issue.
Operand code space is almost always at a premium, so will be "spent" first on supporting the operations most likely to be used. NOR (at least with an immediate operand) does not appear to make the cut.