Many websites that have long operated, but zero PageRank with high traffic Alexa. To get the Google PageRank is not easy because the measurement using the algorithm to consider "more than 500 million variables and 2 million term" (Google source).
PageRank related with many numbers link your site to other link, where the page always appear on the top search engines and needed the right keywords with relevant contents. PageRank not real traffic like Alexa ranking but PageRank only algorithm how important the page for everyone.
Alexa ranking is real traffic, where to improve alexa ranking can use other services that provide traffic, such as directory website, buy traffic on google adwords or others service. Higher Alexa traffic does not a website the index by google. So, PageRank and Alexa both very important as indicator the website needed lot of user anytime, anywhere and get it as soon as possible. Good luck.
More about PageRank : Wikipedia