I really don't think a single answer or factor is the magic answer, but I will rank them and comment on each:
Are bigfoots too rare?
I am very confident that these creatures are very rare. I really don't think there are many more than 5000 continent wide. Considering their huge range, which also holds approximately 1 million black bears, this makes for extremely low density. This is probably the primary reason why they are hard to catch.
Are they dying out?
I suspect they are, very slowly. I suspect their density was higher 1000 years ago, but we may never know that for sure.
Are they migrating north to Canada?
I believe they are becoming less dense in the American Lower 48 just like most larger mammals. As the human population increases, other large mammals are under increasing pressure. Canada, especially 150 miles and more north of the US border, holds many fewer humans and is a safer environment for larger mammals.
Do bigfoots have super keen senses and brain power?
When American immigrants met native Americans on the push west, they believed Indians had almost super keen senses in the wilds. Compared to city folk, they did. In a similar sense, modern man does not have the same senses that a prehistoric hominid would have in the wild. I believe in the case of sasquatches versus Homo sapiens, this is the case.
Do they hide in steep mountain hide outs?
This factor is closely related to their intellect and ability to survive in the wilderness. Yes, they probably inhabit the most wild areas that have no Homo sapiens there. There are probably more than one reason for this too. There are likely to be more resources for them to eat, and fewer humans to hide from.
Do they detest humans which motivates them to hide better?
I don't think it's as much hatred as it is fear and wisdom. All animals hide from us, Because we behave like super predators. However, park animals who do not see our predatory behavior, so they behave as if we are tourists, which we are.