Index page vs KW specific page

google ranking main page not keyworded page

[mr_tim]

I've been pushing a site with one keyword per page, yet on the most competitive keyword google insists on ranking index page rather than the page optimised for the page.

The keyword density on this page is very low ~4%, so is the SERP attributed to backlinks linking just to the index page rather than the specific page?

How can i improve the SERP? As it is not the main page for the keyword so bumping up KWD will be out of place.