Have any of these people said why they have made that choice?
I don't use twitter, but one possibility might be that it actually isn't a discussion forum. It's a place for drive-by firing off of thoughts. For a prominent person, the function of a tweet is to say, "This is what I am thinking about at the moment," so as to invite conversation elsewhere with the people they already know and find worth while talking to. This is far less time-consuming than an actual discussion forum, where it's expected that a post will be of a more substantial length and that you will participate in subsequent discussion.
I predict from this hypothesis that Eliezer makes hardly any replies on Twitter to replies to his tweets.
If you want to explore the community more, I recommend reading the Library, checking recent Curated posts, seeing if there are any meetups in your area, and checking out the Getting Started section of the LessWrong FAQ.
The Open Thread sequence is here.