Why is this page rank much lower than other similar site

I think you mean SERP ranking, not page rank. Page rank or PR is commonly taken as toolbar PR that ranks from 0-10 and is mainly dependant on amount of backlinks you have. Page rank of the sites linking back to you also have a major influence in your own page rank.

SERP ranking on the other hand is the organic search result where you search for your term and your site shows at #3 or #9 or so. You mentioned that your site ranks far behind your competitors that have similar sites. Well, they might have similar sites and similar niche, but I bet they have more backlinks than you have or some might have more quality backlinks as well. Its not only onpage optimization that matters, number of quality backlinks are also very important factor in getting your site ranked on organic search result

My advice, based on my five years of experience:

Continue to: Get backlinks, update and remove outbound links not working.

One of the factors of the engine algorithms, that keeps a web page at the top, is percentage of returning visitors based on the web page's rankings. This goes to say that having a useful resource, for your keyword phrases, is a good focus.

Better get rid of all the unnecessary backlinks. SEO are the one can enhance the page rank.