My site has reached the top 10 in Google for article directory, at 5, and "article directory" at 2, which is great. Thing is, I have no idea why. I have made no attempt at SEO and there are many sites listed below mine with vastly more content. If I knew how I had managed it, I could duplicate it.
You have 12,000 BLs (yahoo) for the home, which makes the home quite strong and a link to a page called "article directory" with the right content on that page.. this could be alot of the reason.
I cant remember exactly how it works (whether its a certain PR or age or relevence). But basically Yahoo or MSN will show many more BLs than G. (somewhere around 10x) this doesnt mean that you have 1/10 the BLs in G.