The web is transient by its very nature. In the time it takes to open an FTP program or change a few settings in a CMS, years of content, years of history, can completely disappear.
Maybe it’ll get captured by Google cache or the Internet Archive. But it won’t be where it used to be, and you won’t find new content along the same lines, either. More importantly, if the site was a place of interaction, that means suddenly you have lost a community.
The result is just jarring. You’re left wondering what happened, and sometimes there’s no explanation. If it’s a larger website, the abrupt transition or closure might have made the news somewhere, but if it’s someone’s blog…
It’s a place where you used to interact, even just a little, and suddenly it’s gone, and you don’t know why.
Whoever took everything away probably didn’t mean anything by it.
But it’s insulting. It demonstrates antipathy towards the site’s community. It says to them: I don’t care that you came here. I don’t care if I ever interact with you on the web again.