I can only guess that it was like, “Well, good people are wealthy people and wealthy people are good people, and we want them in office, so we have to facilitate their coming into office. Why would we think about some system, like giving them a tax break, to get rid of their financial conflicts? Let's just let them have the financial conflict of interest. Keep it secret initially. Now it's disclosed, but it's still there.” They design the act so they can still do whatever they want.
What can I say? Politicians write the rules for themselves. They wrote rules that work for them so that they can profit from their decisions. No party has stood up and said to stop it for everyone, because it also has to be for opposition party leaders, who are almost as powerful in a minority government and who can introduce bills that can pass because the opposition parties hold the majority of seats in a minority Parliament. It has to be on a sliding scale, but you have to remove these financial conflicts of interest for everyone if they have any decision-making power at all.
I don't know. Again, you would not be able to find the discussions as to why these loopholes have been left open since 1984.