The slightly cynical answer is that the tax avoidance is something that requires a continual fight as people come up with new schemes, and is often left alone so long as it's only used by a small number of well-connected people. For example the "K2 Scheme" was becoming very popular among all sorts of people from ordinary backgrounds who'd done well, so it got into the newspapers and action was taken.
A systematic review of how the dependent territories relate to mainland Britain, how power is distributed in the UK, and how the tax system should work to discourage them from becoming tax havens, is something that's basically beyond the capacity of the UK political system to manage. The tendency to leave old arrangements alone or tinker with them rather than putting the state on a systematic, democratic, level footing is simply too strong.
People would ask all sorts of awkward questions like "why is the Isle of Man 'self-governing' but Scotland isn't", "why doesn't Northern Ireland have a functioning legislature", "what would an English Parliament look like", "what are the Barclay Brothers doing with Sark", and so on.