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Because those pages is either not important or similar to others pages. Search engine will give more weight to those pages which have low bounce rate, good content and high click through rate.

What will happen, if a site have million page e.g. forum, gallery, buysell portal, search engine index all and return as a search result to user? The answer is, those result will not relevant to reader.

Especially on smaller sites, Google will index most of the pages when it finds them - providing you have a good internal linking structure on your site. If, however, as time passes, Google finds that these pages never change and nothing is linking to them, the "value" of these pages will drop in Google's eyes and they do stand the chance to become deindexed.

If you want them listed, build some links to them.

My experience shows that Google indexes in two stages:

1. Initially a large number of pages are indexed. The more links you have into the site, the more pages get indexed.

2. After a few days Google applies filters to the content and removes pages that, as Andrew said, "don't shine" -- either they get no traffic or Google's algo removes them for being low quality or whatever.

It makes sense to wait a few days before applying the filter, since as a search engine you don't want to risk some late-breaking news getting hit by the filters and not appearing in your index. It's safer to apply the filters after the fact, not to mention it probably takes a while for the filtering to catch up to all of the new pages being indexed every day.

My 2c.