Because it's not a perfect solution. It's been awhile, but I remember hating the post new bookmark interface, because it was hard to find, and too lengthy to use (the toolbar, as you can imagine, wasn't a great choice). Sometimes all I wanted was to plug in a link and click submit. I remember trying to delete a section of bookmarks (which was all of my bookmarks at the time, since I imported them and changed my mind) and having to go through them page by page, selecting all of them, and then deleting them. Which was wasteful, and as I didn't have the keyboard navigation I have now (Chrome for Linux didn't exist then), it was also painful. There was way too much text on the page, and this upset my Asperger's brain. I need visual aids in the way of graphics, and too much text leads to mental overload on my part.

(I could go back and point out more things that were unaccessible to me, but I don't know how much detail you want. All I can say is that I'm not a fan of delicious' attempts to browse -- the lists, like the AO3, become unwieldy, and the 'bundling' is awkward to use. I remember trying to set it up and getting frustrated at how much clicking was involved.)