There are a number of reasons why this is so but, for me, an important reason is that the more prescriptive the Code, the easier it is to get around. Rules are easier to arbitrage than principles. The more prescriptive and the more precise the Code, the less people will think about what they are doing.
If it’s principles-based and less prescriptive then market participants will have to think about whether their actions are consistent with the principles of the Code. The Code is not a procedures manual. Rather, the Code articulates principles that need to be taken into account. Individual firms may then take these principles and reflect them in their own procedures manuals.