I think people like having a space where they can develop their own identity / brand.
Internalizing gains is desirable, so it's not a surprise if people (such as me) might principally locate their posts on their personal sites or blogs and not post them here.
But it doesn't necessary explain why they don't cross-post. I haven't found that to be much of an obstacle.
Ever since Eliezer, Yvain, and myself stopped posting regularly, LW's front page has mostly been populated by meta posts. (The Discussion section is still abuzz with interesting content, though, including original research.)
Luckily, many LWers are posting potentially front-page-worthy content to their own blogs.
Below are some recent-ish highlights outside Less Wrong, for your reading enjoyment. I've added an * to my personal favorites.
Overcoming Bias (Robin Hanson, Rob Wiblin, Katja Grace, Carl Shulman)