Why won't the UK, Ireland etc CTA countries join the Schengen Zone?
There is one fundamental difference between the Schengen countries and the Common Travel Area countries. The CTA is protected by a very effective physical barrier, the sea. The nations in the CTA have found it sufficient to operate a tight border policy and then to have relaxed controls within the countries. UK citizens do not need to carry identification papers at home and they do not need to register their place of residence with the police.
Continental European countries share long and uncontrolable land borders which have few natural physical barriers. For immigration control, the nations in this area rely much more on registration and the ability of the police to check a person's papers.
The British authorities feel that, to join the Schengen zone, they would need to impose continental European style registration laws and make all people carry official identification at all times.
They reconise that to do that would raise all sorts of civil liberties issues in the UK and would just not be accepted. The spectre of the poll tax riots looms when such matters are considered.
Oddly, it is this situation that makes the UK such an attractive destination for Eastern Europeans seeking to live illegally in the West. It is much harder to do this in the Schengen zone than in the UK. In the UK, once you get in, it is relatively easy to disappear and stay hidden.