Although I don't have the "official" and definitive answer, (and someone who does will probably address this momentarily), here's my guess.
A large number of posters here have either stopped posting and had their accounts become inactive, or fucked up and got banned.
These long-gone posters made posts that became important and integral parts of threads that are too "good" simply to delete, but for various reasons it isn't needed or appropriate to include the actual (former) poster's user name.
So, rather than delete the posts, or threads, their (inactive or banned) user names have been changed to *donotdelete*, allowing the content of their posts to remain embedded in threads that should remain as part of the permanent LTTD archives, while the user names themselves are acknowledged as irrelevant.
Think about it.