Bc some Christians put a lot of effort into making this death to self thing happen or making Christ living through them. The problem is the effort, and it comes out as another version of humanity.
In Colossians, by the time you get to the expression 'put these to death' you are so saturated by what was done in Christ, that the sense is sort of inevitable. It is the neo-Judaism there that puts conscious effort into restraining itself and is unnatural and ineffective.