Is it really? I think all these derisive comments against CL are focusing on tangential details to the classified ad market.
If I place a classified ad it is to find a buyer for the services or product I'm trying to move. CL currently does that better than the competition. Yet the comments I see being handed around are about UI, design, automation and other items that might, but don't actually increase my chance of selling. To replace CL those sites need to provide the person posting the ad with a chance of finding a buyer that's equal to or greater than CL.
If those new start-ups can't provide that, but only have a shinier UI to offer, then they're competing on irrelevant details that aren't the main drivers behind people's decisions to post/view CL.
Others have mentioned marketing approaches and techniques to help improve those aspects. But most of what I've read is summarized as: "we'll provide a better app/ui/api" and that's a noble goal, but the reason for existing of these sits is to connect seller with buyer, what do those things bring to the table if a new app lacks a sufficiently large viewing audience? That's the problem that needs solving, not the app/ui/api one.
Padmapper is the Mozilla.