The corporation tax regime is a UK-wide system and introducing different rates in one part of the UK could distort competition.
Many companies trade in different regions of the UK. It would not be easy to determine the proportion of their profits liable at a “devolved” rate. In addition, such a measure could create opportunities for some companies to manipulate the rules in order to benefit from the lower rate. That might well result in a system that would place a significant administrative burden on both businesses and the Government.