This is pretty sad, but it's also pretty clear why it's happening. From the official announcement:
... the internet has changed. Large sites like Facebook do all they can to keep readers on their network, rather than sending that traffic out to individual websites. As such, many readers - who used to visit dozens if not hundreds of websites a day - now visit only a few sites ...
It's kinda like Wal-Mart coming to a town full of little mom-n-pop shops, only internettier. Those "large sites" include Reddit, Instagram, Imgur, Twitter, Tumblr, Webtoon, and Tapas -- the main ways people consume webcomics now. Few people visit a webcomic's dedicated site these days, outside of established behemoths, and all those Project Wonderful ads go largely unviewed. If fewer people're seeing the ads, buyers bid less for the ad space. Revenue falls. The company loses both its lifeforce and its raison d'etre.
Project Wonderful did a really good thing for a really long time, and helped a lot of creators get off the ground. I hate to see them go, but it isn't their fault. It's the apps and startups and "walled garden" models dedicated to hogging all the traffic and ad revenue for themselves, while sorry hopefuls like us produce their content for free.