It's a very intriguing issue. It's not just citizens, of course, but in some cases it may be people doing business with government; it may be a whole range of people who will come into knowledge of what they consider to be a potential conflict.
I can't think of a principled reason that we wouldn't want to hear those concerns. On the other hand, if you could simply go to all the political opponents of a particular minister and say, “Why don't you start flooding the ethics commissioner with complaints day and night?”, it would become an untenable situation, a more partisan situation, and one ultimately for which there aren't the resources to do justice to the meritorious complaints.
I think a model in which there is an opportunity to welcome complaints from others, but also a screening mechanism such that the commissioner can decide which ones are meritorious and not necessarily have to investigate every one, or every one in the same way, would be a middle ground or a balance.
If you're looking at it from the standpoint of the purposes of the act, I can't think of a principled reason that you wouldn't want to hear concerns from citizens or other interested parties. At the same time, if you opened it up without any constraints, you would undermine those very goals.