So the rumor is that Hulu kept exclusivity for some of Adult Swim's shows, but ever since the AT&T merger, and with the launch of HBO Max, Hulu is a competitor. So they don't want to create new content for their competitors, so shows that aren't HBO Max got axed (Including Mike Tyson Mysteries, one of my favorites.) Of course they paid a bunch of money to get rights to non-exclusively show Rick and Morty because they know where their bread is buttered, but Venture Bros. didn't quite have the following to be worth the money.
BUT there's still a chance that HBO Max could get Venture Bros. exclusively if it changed to a different name. Apparently this is why Mr. Pickles was changed to "My Mama Called Me Sherriff." This might be why Adult Swim tweeted that they're working with Doc Hammer and Jackson Publik to keep the story going.