Because your page is one of the 400 pages the user liked. Some of those pages are hyperactive as far as new content (New York Times, CNN, etc.) How would you prioritize the stories?
It's just apparently not something majority of their users need based on their usage patterns. If they switched to this as default, brands would complain as well, as a post appearing at 7 am is very unlikely to be scrolled down to by a user who checks Facebook at 8 pm their local time zone.
I have that bookmarked in the browser's toolbar instead of default, but you're right, ability to default is highly dependent on how you reach the facebook.com page.
