Why there is no PR on directory pages

I found that many of directory pages containing a few outbound links got no pr from google. Even with the dmoz and yahoo directory.You will see some pr in the category listing pages but if you go deeper into the site listing pages, most of the time you will not see any pr on them.

What is the theory? If it's about the number of outbound links? Of if it's about the number of clicks from the root page? Or anything else?

Google has very little respect for directories nowadays. When sheer number of links was enough to get you great rankings online directories were abused to hell so the huge majority of them have been devalued so far as to be pretty worthless. If you look at them mosat have pages just full of links buried deep in the site. As this isn't going to increase the user experience, which is what Google want, they don't assign them a value.
One great way to get some decent and valuable inbound links is to use article submission sites such as prlog, I have 2 PR2 articles and 1 PR4, all within 6 months. If you search around I'm sure you will find many more good sites. Because quite a few of the article submission sites are still human checked before your article is approved Google seems to trust them more, if you look at the vast amount of crap in the majority of directories then you can see this is not the case with these directories.

The point is that I'm finding a better approach for link exchange, I think that it's useless if we'll keep putting the reciprocal links in a link pages with no pr at all. So I would like to find out what are the factors to make the pages to have no pr.

Confusingly, according to the Hilltop algorithm, it's good to put our link in an expert page. And one of the condition to make a page to be an expert page is that it must contain a few outbound links to other websites on the topic. This seems to conflict with the fact that google tend to give no pr to pages with a lot (or a few) of outbound links.

Any idea?

Well consider this fact, PR usually on internal subpages was passed from main page - unless links exists. That was before. These days i notice google remvoed PR on internal pages passed from main page, so only good way to increase PR of internal page is by people linking to you. So this means all directories have lost internal PR on internal pages.
It has become much harder to maintain PR on internal pages, beside the main page.
But as many people say, do not look so much at PR.