Here are some reasons:
It's quick and easy. No fancy tags to learn, only a few simple text "hints".
It's powerful. While you can edit WikiText without using any DocBook at all, you can also use any DocBook tag inside it.
It versions. A complete version history of your edits is maintained in the database. You can revert to a prior version if you didn't like something you did. You can do this also with cvs, but it is much easier in the online system.
It shares documents. Authors who work on documents with other authors can collaborate through the WikiText. Yes, cvs also does this, but again WikiText is simpler.
It's accessible. All you need is any web browser and an LDP Database account.
It's WYSIWYG. There's a "Preview" feature, so you can click the "Preview" button and see how your document will look on the LDP site. There are no utilities to run, nothing to learn, no DTDs to install or catalog files to munge with. If you've ever tried to get a working DocBook system on your machine, you will appreciate this! :-)