Have any of these people said why they have made that choice?
Not that I've seen, but I have seen Paul Graham tweet complaints about Twitter, and I think I've seen Eliezer complain about the behavior of users on the platform.
Twitter makes tweet replies less prominent than Facebook. When I'm scrolling down Eliezer's Facebook feed, I see comment replies to his posts. When I scroll down his Twitter feed, I have to click a tweet to see replies. So that could be playing a role.
It doesn't seem socially expected to reply to replies to your 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.