I think it is as simple as an algorithm that rates how frequently new updates show up vs how many people visit a site. If you rarely make updates to your site,

I think it is as simple as an algorithm that rates how frequently new updates show up vs how many people visit a site. If you rarely make updates to your site, Google crawls it infrequently and new things won't show up very quickly. But if you do have frequent updates and lots of traffic, like any popular forum style site, you will get lots of crawler traffic. And I would bet the algo does the same for all endpoints on a site. So the "about us" page on a popular site probably ends up not being crawled nearly as much as the new threads page.