I think it's all about craigslist's ease of use. there is no 'required' information that you have to input. Take cars for example.. you don't have to spend 20 mins filling out a huge form that contains a bunch of [possibly] superfluous information. you just post whatever you want to post and that's it.

try to make anything "better" than craigslist almost requires that you force people to provide more information and make it less likely for them to use your service