The code recognizes that for large wire sizes there are reasons of flexibility and handling as well as availability that justify building a high amperage link out of parallel wires. For smaller wire sizes, the code considers the problems related to paralleling to outweigh any of the above considerations and therefore requires using the appropriate size single wire.
There are issues of uneven current distribution, inductive coupling, labeling, fault detection and more that all make use of parallel conductors less desirable.
