The same is true (perhaps moreso) for audio/visual media online.
[1] It takes someone else to first know about archive sites, consider archiving a link pre-emptively either manually or in an automated way, which doesn't always occur. Then one problem is if a link changes a version may have been archived at the old link but the new link doesn't show as being archived (sometimes redirects are in place, other times not). Another issue is archive.org respects robots.txt and webmaster requests so crawls can be retroactively made non-public (archive.is is a bit different but there's so little known about who operates it that it's unclear how reliable it is for long-term reference).
Why content preservation is not very popular among webmasters (why NH and 1000000s other sites do not do it?) is an interesting question for Internet philosophers.
Maybe, because people mostly want the shit to sink and not come up again?
Also, why web-archiving is biased right-wing?