Well it depends on what one wants to focus on. While the Yukon is spectacular and has many animals I am not sure any case can be made that there are more to be sighted there than in Alaska.
British Columbia is beautiful but the part that you travel though to my mind is not as scenic as either the Yukon or Alaska for sure. Therefore for me I will spend more time in the Yukon and Alaska.
I have driven it before and lived and worked in Alaska so I may see it a bit differently.
I think for the majority of people going to Alaska the goal IS ALASKA. With limited time they probably want to focus on their goal. I cant fault that reasoning.