Also, LJ groups have not been successfully replicated by any of the other major social media networks.
Wordpress does the same, which is why I primarily use it and not the innumerable alternatives.
I think the desire to do long-form writing, especially regularly, is a relatively rare one, which is why we see most people migrating to sites that mostly do images / sound bites / etc.
But the people who do do long-form writing will probably always be with us and probably always need tools appropriate to that genre. I don't know much about the history of LJ or why /how Wordpress supplanted it for long-form writers, but I do know that WP works pretty well for me, or at least well enough that I haven't seen any alternatives good enough to consider switching.