Because if you are ranking well (not just talking about appearing in the serps - talking about page 1/2 - 'WELL') you're pages are obviously well written, compliant in Google, receiving links/pagerank, and will eventually display a pagerank on the 'foolbar'
If you know so much about it, why were you asking the question in the first place? I know rankings and pagerank have nothing to do with each other - they are completley seperate algorithms. PageRank algorithm is leased from Stanford University and the Ranking/Quality algorithms use several hundred factors in their calculations which they file dozens of patents for every year to improve.
PR of a website does not have anything to do with the ranking of that website. It depends on the quality of links, quality of content and other associated factors.
PageRank calculation has nothing to do with quality of content - it's on a completley seperate algorithm, you think it cares about your content/words etc?? It is a link graph score, but a hell of a complex one with billions of pages to calculate. It's the ranking/quality algorithms (Googlebot) that look at your content, and decide if you are duplicating, linking out to bad sites etc
The only way to get a pagerank 'high' is to achieve links from pages with either equal to, or greater than the pagerank you wish to achieve. That's the formula I've stuck to for a long time and it's proved true for me. E.g. generally speaking you'll need a link from a PR7 to get a PR6. If you want high pagerank go and get some high pagerank links! You could have a pr6 if you get a couple of pr6/pr7 links (depending on the number of outbounds) - but as you and I know, it won't help your rankings one bit as it's keyword inspecific - it doesn't look at words.
Back to your original question which was 'which is more important pagerank or alexa' - you don't need to worry about either?? Get the rankings for your site(s) a simple call to action/point of sale and you'll make money?