What are the ethics of answering your own questions in a regular question/answer format?
When asking a question you can click on a little check box at the bottom that says "Answer your own question - share your knowledge, Q&A-style". This shows that answering your own questions is encouraged. There is even a badge that is called Self-Learner to encourage this. If you want to help others by providing an answer to a question that you had where you couldn't find any help at first any you had to search for the answer yourself you are encouraged to share your knowledge so that others will have an easier time in the future.
What about people who don't use the community wiki box?
You can create a community wiki, but community wikis are a weird tool that is not always perfectly understood and has changed quite a bit since it was first devised. In the beginning it was to give people a way to participate earlier, as there wasn't a "suggest edit" privilege at first and using a community wiki reduces the amount of rep for this.
For example here on Meta you would need 2.000 reputation to edit something, but with a community wiki you can edit as early as 100 reputation. This helps people with low rep participate - but these people are not always that interested in helping some greater good or simply know about this. Community wikis are rare, so encountering them is not happening too often and then knowing how to interact with it and knowing about the specific topic is not always happening.
Using the community wiki to answer is not really that important. It might be the nice thing to do if you combine the existing answers and it might be the normal way to use a normal answer if you go with a completely different approach, but to be honest: it doesn't really matter that much. You can answer your own question and you are encouraged to do so if you feel that your own answer is the best one.
There are lots of good, high-voted answers out there that simply sometimes don't cover everything, and a canonical-answer bounty seems sometimes excessive. And if you use the community wiki, for some reason community canonical answers don't get as many votes as the incomplete ones. Are there any ways to solve this? Why does this happen? Why don't people contribute to some of the community wikis?
I don't think a bounty is excessive. That's what the bounty system is for after all. I am going to still remind you that answers should answer the whole question. If they don't they are not an answer.
But if you had several valid answers and you wanted to combine them by creating a tag wiki answer that's great to show future readers your approach and a possible solution to this problem.
But few people are interacting with these community wikis for the aforementioned reasons and because the interest in your question has waned. Questions that are away from the frontpage and not in the featured list (meaning they don't have a bounty) are less visible, so only a few people interact with them from time to time when coming across them. This is especially visible for questions that were in the Hot Network Questions and attracted lots of attention from outside the site's regular users.
Furthermore the community wiki answer is probably quite late to the game if it's a collection of previous answers and has therefore lower votes, meaning it will be far down in the list of answers in the normal ordering. This means only a small percentage of the people who see the question will see this answer. Most people look at one or two, maybe three, answers as long as they are not especially interested in the question. Everything further down is often more or less ignored by the vast majority. Most people also don't come back to a question once they have looked at it once. Maybe if they answered they are reminded of it when getting an upvote and look through the other solutions, but not the majority of people.
Even if you accepted your own community wiki answer it won't be at the top. If you accept your own answer this doesn't affect its ranking. Only accepting other peoples posts puts them at the top, before the normal ordering.
The Gamification component of having Points-Badges-Leaderboards is also important. Most people want to get reputation points for their effort. Interacting with a community wiki doesn't give any reputation and is therefore lower on the priority list for most people.
There are not really any ways to solve this. Putting a bounty on the question and mentioning the answer is a way, but not the most effective one. But it's also not really a problem. You've got your answers and maybe helped others by mentioning the solution you went with.