It is clogged with spam. Clever types of spam, often ones that crowdsource their content by mining previously legit posts. Since there are no reputation mechanisms to speak of, it's far too easy to waste your time with stuff that's either spam or a scam.
Meanwhile, data visualization has progressed dramatically in the last decade. craigslist remains in 2001. Wading through apartment listings, for example, is miserable, and there are almost no mechanisms in place to encourage that very basic information is communicated by people listing their properties.
craigslist can keep its hideous, serifed-out visual design as long as it wants. The big beef here isn't how craigslist looks – it's how it works.
The fact your dog doesn't shit on your pillow doesn't excuse his taking a dump in your shoes every day.