Pacing, budget, focus, general irreverence.

The Venture Bros. is a fairly specific satire of Johnny Quest and other kids-versus-supervillains media. It’s grounded, and being grounded is half the joke. Ultimately these characters exist in the mundane world and just act like total weirdos. The comedy is vicious, but often bone-dry. It occasionally gives episodes to tertiary characters to remind us that even henchmen have lives. The show has that Seinfeld quality where everyone is kinda broken in a predictable way. Growth is possible, but even when that doesn’t lead to killing off a happy character, it means the stakes and consequences of each episode have to be realistic.

Rick & Morty is a freewheeling multiverse that can do anything whenever it wants. It was created by two guys who honed their comedy on a platform where being the funniest thing of the month secured one more episode. There’s no laugh track, but the naturalistic dialog is relentless. Keeping most episodes centered on an asshole scientist and his hapless grandson simplifies the hell out of the audience experience - their bickering answers questions. Which is very helpful, because the show will invent and then blow up whole planets, including ours.

Basically, the closer Adult Swim comparison for Rick & Morty was Aqua Teen Hunger Force.