It's also better for community cohesion if we all post in the same place.
Due to the fact that they could easily post here and receive all the benefits you listed and more, my perception of the problem is not that they do not realize that there are multiple benefits in posting on LessWrong rather than their own blogs, but that there's something about LessWrong that makes it an undesirable place to post.
My top guess on that, based on the talk that I've seen going on around the place, is fear of criticism / rejection / negative karma. See also.
Is community cohesion something to aim for? Ideally, rationality should be the baseline, not the marker of our particular community. As recent threads on gender have suggested, there's room for quite a few different communities which share core principles. Ideally, lesswrong would be one of many places where rationalists of different stripes feel comfortable. Of course, the question is whether it's better off huddling together for the moment in hopes of reaching critical mass or if we're already past that phase.
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)