There are probably multiple reasons. One that I remember him giving was that on Facebook he can simply ban people he doesn't like in a way that he can't here.

The nature of feedback was his explanation if I remember correctly. More friendly reactions on other places. Also, on his private pages he can simply remove any reactions he dislikes without worrying and debating about whether he is abusing his powers.

I think that LessWrong does not feel for Eliezer anymore like "just me and my friends" (unlike e.g. Facebook), but more like a public place where he is judged, and where people are waiting like hungry sharks (my metaphor, not his) to get some contrarian points by finding some flaw in something he wrote.

(Also the online bullying on RationalWiki about "basilisk", where he will be forever criticized for removing some comments years ago. If he removes comments from his Facebook page, there will probably be much less drama; maybe.)

Simply, people are behaving less friendly towards Eliezer on LessWrong, and more friendly at other places... so, you know.

Here you go. ETA: oh, you asked why. I don't know. I note that he's gone for some months without posting new threads here in the past, though this is certainly longer.