The final season for Regular Show starts Sept 26. Strangely, it's a burn-off. We would think Cartoon Network would end Regular Show's run on a nice note, but the series new episodes are going to be burned through with a new episode every night, at least on week days. Cartoon Network does this practice every so often, but usually with WB animation based shows it's forced to show by Turner/WB higher-ups. It has shown very little love to these shows, putting them at odd hours or just not being direct with fans and viewers about their status.
But Regular Show is a Cartoon Network original. It's odd they wouldn't want to show it on the network's featured Thursday night slot with their most beloved cartoons.
We can only have theorize, but most likely think it's out of either a disregard for the show or some behind the scenes action having to deal with ratings or contracts. In any case, it's somewhat ridiculous to not try and grab as much as they could for the final season in ad revenue or spectacle by releasing it along a normal schedule.
It's good for us fans though as the series has ended production anyway.